I always pace myself with how many primary party pokemon I have.
I usually go through the first few routes with only my starter, and then by the time I beat a third gym I usually have picked out two more pokemon that are early strong guys. Then I might go awhile with just those three and eventually add the next two, until finally a legendary pokemon rounds out my party just prior to the Elite Four.
I always end up catching a couple of extras on my first run through. I'll see something and go "ooh! that looks cool!" and to hell with how many pokemon I have. If I want it, I want it, and usually end up with nine or ten pokemon by the Elite Four - at least two of which are usually left behind, level-wise, but that's life. My White team ended up with nine. I had to shelve one of the pokemon I planned on catching, because the pokemon I kind of anticipated ditching for it turned out to be the MVP, and is currently my highest levelled pokemon at 65. Excadrill, too, shunted someone out of the way and is currently at 62 - so at least the replacements are clearly doing their jobs well.
On further plays, I tend to be more frugal. Only catching the ones that I know I'd really like.
The complete lack of knowledge about Black has my usual trend in a tizzy though.
I'm about to beat the fourth gym and my party is as follows.
Tepig (31)
Munna (31)
Gurdurr (32)
Krokorok (32)
Ducklett (25)
I currently have ex. share on Ducklett and am bringing him up to speed.
Ohhh, I'd call that a good team! Balanced, for sure. Didn't realise you could get Ducklett before the fourth gym, but my brother has one (now evolved, natch) and it's very cool. I really want to get a strong Munna, too. I think it's dead cool. I'm actually finding I don't like Reuniclus nearly as much as I was expecting. Was anticipating him as the MVP, and yet he's now verging on sliding into disuse. Wonder if a Swoobat or Musharna would fill the gap. Currently got a Klingklang that's chomping at the bit to get involved, but I don't want it on the same team as Excadrill.