One tip, posted many moons ago by a wise forum poster named Bernard, is the 7/20 rule.
Essentially, for any difficult passage, you should practice it (slowly) in 20 minute sessions, with breaks in between. If you aren't making significant improvement on the passage after 7 dedicated 20 minute practice sessions, you're playing too large of a passage and need to reduce the amount of material you're practicing.
If you can, order a method series. It'll give you good technical basis from which to start on grade 1 pieces.