If you get stuck on your rival you should get as far as you can and when he sends out his first monster write the level of it and keep doing that as far as you can.Then build your pokemon higher than his and victory!
When you'r e strong and you have a weak pokemon in level goto Viridian Forest and you will find some metapods that are at level 6 and give your pokemon 61 experince points.
When you're going to trade with a person in the game without a link cable, when he or she tells witch pokemon to give, say no and build that certain pokemon to a high level and then trade and you will get the pokemon at the same level as the pokemon you gave the person.
Remember that you only have one chance to capture articuno,mewtwo, moltres, Zapdos and you have two chances to capture snorlax.
Go to the digleet cave to build up your pokemon. I built my level 10 geodude to level 25 in the Digleet Cave.