what are frugal tips/sites you kick yourself for not knowing about before? reference thread

hi all, i recently found out about two frugal sorts of things i wish i had known way earlier:

1. amazon prime store credit card, where you get 5% back on everything you buy on amazon as long as you have a prime membership. (i know amazon visa card just upped to 5% back too.)
2. cashback sites like ebates (recently joined ebates to get 5% back on a site i use a ton, plus got credit card to get an additional 3%.) i am going to check out the other topbachbacksites too.
3. citi double cash rewards card (for any non amazon shopping i do, 2% back on everything.)

can any of you please post your favorite tips (so i can join now too instead of kicking myself a year from now?)

i joined reddit a while ago but hardly ever use it so i'ma newbie, hopefully i'm posting right. i quit my day job about a year ago and hope to become like a frugal ninja :D i love saving money. ps i am not affiliated with any of the cards/companies mentioned above.