well, there's nothing to do to make sure you get the right nature more often, but you can try, if there's a particular speed you want it to have, to have something slightly slower than the target. ex:
say you're in emerald, and want to catch a groudon with at least a 25 IV in speed (148 stat at level 70 with a neutral nature). raise a pokemon so that it has 147 speed (or as close as you can get without going over. vitamins might help you here), and send it out first. have it use a move, then if the groudon's not faster than it, reset.
also, if you're really serious about getting a groudon with a good nature, use the master ball. instead of having to spend a long time catching each one, it's about 30 seconds from soft resetting to checking its stats. maybe a little more, but it saves a lot of time.
that's how i got an adamant groudon with ~26 in speed and attack.
