um actually doing pushups WILL help, to a certain extent.
the pects (chest) and triceps are antagonistic to the biceps.
Take another example (rare but happens):
Someone does leg presses the whole day, as a result his quads (the front of thighs) are HUGE, but his hamstrings (back of thighs) are minute by comparison. Whereas size is not a -very- good judge of strength, here it means that his hams are limiting the improvements in his quads, because they are antagonistic to each other.
So basically doing pushups, depending on which variation you do, adds flexibility to your biceps and lats. (So don't slack and go down lower! :p)
If you are a zero fighter right now, don't fret. there are 'easy ways'. Just get a indoor pullup bar. everytime you pass by it, do a few. if you can't do any, its okay, just pull to the maximum you can, and HOLD IT THERE as long as you can.
Another method is assisted pullups, but those are dangerous (if you are exhausted and the other person isn't paying attention.... gd luck to both of you.)
So i suggest what some bodybuilders do. "negative training" This involves getting to full pull, aka chin over bar first. (via any method, assist, jump etc) THEN, you SLOWLY let go, taking as long as you can to reach the full stretch. be especially slow during the first and last parts: those are the hardest, and therefore you train the most.
(btw for those who are looking for one, most larger sports stores have one - you can screw them onto the doorframe, just be careful: i fell off mine before.)
another tip.
for those of you who see your school canoeists that do pullups in a 'wave' like motion, DO NOT LEARN FROM THEM. that method does a few things:
1) utilises momentum to literally bounce you up again after reaching the bottom - if its easier, you train less, its that simple.
2) you don't ever get to full stretch using that, so your forearms (not the finger muscles) don't get that much training. so you lose flexibility and arm strength - ever notice how HARD It is to get from full stretch to chin-over-bar?
3) wreaks HAVOC on your joints. everytime they bounce, the joints take the strain. you do not want your shoulder burned out.
4) increases risk of tearing just about any muscle. one person i knew half-sprained

his back
so just do the full motion during training. if you want to do any 'techniques', save them for the actual test, if at all.
after all, what's the point of doing '20' and then when you get to the IPPT, the niao instructor makes half of them 'no count' and you end up with 10?
and finally - consistency, for <5 pullupers - just keep doing, especially if you haven't enlisted, since you still have time. just TRY once or twice a day and you will see improvements - and it takes <2 minutes total... why not?