HT Games
Levels 1-20
Following the plot through the earliest stages of the game usually provides enough EXP to continue, but there are a few areas to concentrate on for a heads up on the competition. Spend a few extra minutes training on Rt. 30 before facing your first opponent outside Cherrygrove, where you should be at least level 7. Take on Faulkner at the Violet Gym for some extra EXP, and continue through Rt. 32, where you'll find some higher level Mareeps and Bellsprouts. Face your rival in Azelea town once you've reached at least level 15.
Levels 21-41
Spend your 20s grinding on the high EXP Tauroses on Rt. 48, just south of the Safari zone. There's also a patch of grass near the waterfall with a remarkable amount of high-value encounters. Then, head to Victory Road and search the grasses guarding the Elite Four. In addition to the forced encounters of the plot line, these grinding spots will have you defeating the Elite Four in no time.
Level 41+
The best way to train your Pokemon above level 40 is to rematch gym leaders and previous opponents throughout the game. Fight the Elite Four repeatedly until you're comfortable with your team, then head to Mt. Silver and train with only a single Pokemon in your party at a time. Then, return to the Elite Four and fight with only a single Pokemon. Repeat this process with each Pokemon until you max out at level 100.
Helpful Tips and Tricks
Trading your Pokemon puts you at a significant advantage, as traded Pokemon earn EXP 1.5 times faster than caught ones. Trading the Pokemon back, however, negates the benefit. The Lucky Egg will also offer your Pokemon a boost of 0.75 percent growth rate. Day care can also help catch a new Pokemon up with the rest of your team.