Everyday that you can, try to...
Get as many rematches with past trainers as possible
Battle any trainers that you've ever skipped and get their number if they offer
Battle the wild pokemon at Mt. Silver(must be patient)
stock up on rare candies (note: rare candies make your pokemon's stats lower is a MYTH. Really the pokemon just don't get any EVs from it, these raise a stat by 1 for every 4 of them.)
Try to battle the Elite 4 again, their teams are tougher and they have Lv.60-75 pokemon
Also, if you're into extreme item hunting, then try to find a Chansey with a lucky egg( it doubles EXP, VERY USEFUL INDEED)
Also, yay for meganium, I chose it too, and its great
Hope this helps.