Your best (and easiest) way of glitching your game is, on R/B/Y, if you talk to the dude in viridian city who catches a Weedle, then IMMEDIATELY Fly to Cinnibar Island and surf along the right coast of the island. Soon a glitch Pokemon called Missingno will show up and give you a glitched amount of the sixth item in your bag.
Also, in Gen 3, there is a small chance that your game will freeze when you use Thunderbolt, Thunder, or run from a wild battle. This is hard to avoid, but it's extremely rare. It's one of the only glitches where you don't have to do something weird to activate it.