I make it my goal to only capture a six-pokémon team for use. Of course I might keep capturing more Pokémon, but I don't use them outside of HM-duties. I also usually won't trade from other games to build a team, but I most likely will for my future reruns of HGSS, DP and White. I try not to use legendaries. I might have at some point in my life, but that's in the past. Regular Pokés are the way to go.
I always keep my Pokémon the same level. When someone gains a level I dump it at the bottom of the list, so that when all the others have gained a level as well it's its turn again. Sometimes the Pokémon might gain odd levels from more difficult battles, so that mixes up the order a bit.
I don't level up my Pokés intentionally. I just go forth at a normal pace, I won't stop to grind at any spot. The only thing that has ever become a problem through this practise are the numerous Elite 4's, but I like the challenge. Beating the champion with seriously underleveled Pokémon is just cool.
I might use the starters on my first few playthrougs, but eventually I will be wanting a breath of fresh air and will build my team without them. For example, I just recently began playing Ruby again, picked Torchic as my starter, caught Seedot and Raltz before Petalburg, ditched Torchic once the two were able to survive on their own (Raltz learning Confusion).
I always try to have a team with equal amounts of genders as well. Three males and three females, or some genderless ones. I wouldn't take a team with three males, one females and two genderless Pokémon, as it would have to be with two males and two females. Stupid? Yes.
Also, this might be a bit off-topic, but I never EV-train. Never have, never will. Saying that a Pokémon with an undesirable nature makes it weak is bullcrap. The only thing I worry about is it's ability, so that I get the one that I want. Nature doesn't matter one bit, as the game is easily beatable even with "weak" Pokémon.