Know when you are paying too much and why. Bottles are worth more when you are trying to buy them than when you're trying to sell them. Don't spend the kid's lunch money on bottles. When you buy bottles buy the best you can afford. Quality always is easier to resell.
Ignorance is expensive.
Educate yourself about what you want to collect. Read everything you can get your hands on. Have fun and remember its the people you meet along the way, not the bottles that are most important.