My serve depends on the drive I get from my legs and if I get tired, it can drain some life out of my heater. Make sure you keep using your big muscle groups, even on your second serve.
If my opponent has a weaker backhand return, I'll just keep serving to it with different pace and spin so that when I finally decide to sneak one to the forehand side, he's leaning the other way or at least not camped out on it.
If you can't consistently serve to a weak side, try to keep your opponent guessing at least a little. If my first serve is a let, I'll hit a different pace/placement because I just gave him a fresh look at something. An off-pace first serve can be very effective when the receiver knows you can hit a flat bomb - they might be too far back to hit the shorter slice very well.
Disguise comes later after you've developed accuracy and consistency, but when you can eventually produce different serves with a motion that looks the same, returners will have more trouble even when they get used to timing your motion. Keep grooving it!