To me, when I see a true 3.5 player (ie will be a 3.5 forever) I picture someone that bears absolutely no resemblance to tennis that would be seen on TV.
If they are fit, they have bad strokes. If they have a good forehand, their backhand is terrible. If they have a good backhand, their serve is a lollipop. If they have good footwork, their swings are slow and without pace.
Reason being is that if they were fit, had decent and reliable strokes, a serviceable backhand and a respectable serve, and any signifigant pace on their shots....they'd be a 4.0 or better.
So, for anyone with an average base of athleticism I would start there: fitness, strokes, serve, and power. If you have 3 out of the 4, you might be better than a 3.5. 2 out of 4 and its unlikely you'll be seen as a sandbagger.