...Continued from above.
If your strings break, they need to be restrung.
If you are not a string breaker, and are using synthetic, or natural gut, or a multifiliment. The general rule of thumb is to restring as many times per year as you play per week. If you live in a cold winter area, and don't play over the winter, then make sure you have a nice fresh stringjob for the beginning of your season, and then restring accordingly during the season.
If you are not a string breaker and are using Polyester, then as soon as the strings stop sliding back into place, and need to be straightened between points, you should restring.
If you are not a string breaker, and you are using Kevlar, restring at once, with something that is not Kevlar, then follow the guidelines for your new string.
Most people don't realize that string performance is going downhill until it is way too late. Because you play with the racquet every day, you don't notice the gradual decline in string performace. It is like if you bought two identical dark navy blue t-shirts. If you put one in your dresser and left it there, and wore and washed the other one every day for two months. The one you were wearing every day would fade, but you wouldn't notice until it became really pronounced because it happened slightly every day. But when you went back to your dresser and pulled out the brand new one, holding it up next to the one you have been wearing every day and you would immediately see the difference.
Only difference with strings is that you don't have a Wife/Mother/Girlfriend to say "When are you going to throw that ratty old t-shirt out?"
How tight to string?
The reccomended tension range on the racquet is there for a reason, and is a good place to start, in fact, right in the middle of it is good. But it is just that, a reccomendation, feel free to go higher or lower than the range if that is what you feel you need. With Polyester strings, I would start out 10% lower than you would with a syngut, and take it from there. You may end up higher or lower from there, but it as most things with string is a process of experimentation.
When looking for your tension, you are going after two things, and depending on how serious of a player you are, you are going to choose one over the other, to be your primary focus.
Depth, yes, I know, broken record, dead horse. Depth, is key, so important words cannot begin to describe it. Hitting deep is to tennis as eating is to living. You need to find a tension that allows you to take a nice smooth even swing at the ball, and have it land deep. Beyond the service line, preferably in the back 1/3 of the court. It should not take effort to hit deep, you should be able to swing easily, and smoothly, and have the ball land in the back 1/3 of the court. String tighter to shorten up your ball, string looser to get it deeper.
Feel, the second thing, some people like the way a firm stringbed feels, some people like the way a soft stringbed feels. Stiff stringbeds, tight stringing etc, are harder on people with arm injuries so that is a consideration aswell. Feel also has a lot to do with targeting, and accuracy, especially volleying. Most feel that stringing tighter helps with this, but I feel having something that you feel comfortable with will serve you better, and give you better accuracy and control than just randomly stringing tighter.
Do you play for fun, and want your racquet to feel good, or do you play because you want to play your best tennis. That will sort out weather you prioritize feel or depth.
Be certain that if you are hitting the ball long, that the problem is not you. There is a difference between driving the ball long, and not hitting a good stroke, and having the ball land when it runs out of steam and drops to the ground. If your balls are floating long at a blistering 7mph, your setup doesn't have too much power, you don't need to string tighter, you need to put that Amex card away, and make your way to the practice courts.
Different racquets, different tensions, same number on the stringing machine
If you string a 90" frame at 60lbs, and a 115" frame at 60lbs, the 90" frame will be strung tighter. The longer the strings, the more there is to stretch and the looser it will become. Kind of like taking a shoelace and a 30' rope and tying them to a fixture. It doesn't take much to pull the shoestring taut, but you need to pull pretty hard on the rope to make it taut.
Same Racquet, Same number on the stringing machine, different machine, different tension.
All stringing machines string slightly differently, some string vastly differently. A high end constant pull electric machine will string much tighter than a low end lockout machine. Keep you racquets going back to the same machine, if you want the same tension every time. Different people stringing on the same machine set at the same tension will give you different stringbed stiffnesses, not that either would be a bad stringer, but some people string looser, and some tighter. So if you are picky, same person, same machine.
Hybrids are when you use a different string in the main and the cross, this started for durability reasons, as players would break strings often, and need a more durable string. The durable string was harsh feeling and low powered, so since the mains normally broke players would put a durable string in the main, and a soft string in the cross to regain some of their feel and power. This is the case with most kevlar or Poly blends today. You use Kevlar or Poly as the main string (The long ones that go from the head to the throat) and syn gut, gut, or a multi, as the cross string (The other ones that are not the mains
Also players hybrid gut putting the gut strings in the mains, and a cheaper synthetic in the cross so that they get most of the benefits of playing gut, but don't have to pay for a full set.
Lately people have been putting a gut or multi main, with a poly cross, going for the power and feel of the softer string, and the spin characteristics of the poly. This is an expensive proposition for most as this is probably the least durable blend that one can have.
A few jumbled together closing thoughts.
If you break syn gut, and multis, but are not ready for poly/kevlar because you either don't like the feel and stiffness, or don't have the batspeed, or have arm troubles. Try Head RIP control. It is a very durable synthetic with ribbon construction. Softer than poly/kevlar, and lasts a good long time for most players.
If you like the idea of polyester with the added spin, and constant stringbed without having to adjust the strings, but you don't have the batspeed to really make it sing, (If you are currently playing poly and leaving it in for a month, then this is you), or if you like the idea of something that performs in a similar manner, but is softer feeling and more powerful. Try Prince Recoil. I was very impressed with it, and think it will help a large number of players. On the downside, it is pretty expensive.
Lastly, if you want to try something, try it. Stringing is about experimentation, and don't let another person, or this handy dandy guide stop you.
If you have only been playing a year, but you want to know what Luxilon feels like, then hey, give it a shot, you won't play your best tennis with it, but at least you will know what it is like. Just keep in mind the things I have written, and understand the things you are trying for fun, and the things you are trying to make you play better. Keep your health in mind above all else.