There's a zillion, at least, possible tips on this. A very personal thing; what works for one person may not work for another, or if it doesn't work it may point the way to a method which does work. You can spend so much time trying to collect tips...that you don't get anything memorized! Trust me -- I've been there!
Here's one I just got from my teacher, which I've not yet tried. Xerox the music. Mark through the passages you've memorized as you memorize them. Might be very useful.
Here are some methods which work for me -- your experience may be totally different. In no particular order:
1. Move the hand positions. I start by moving RH up and octave, LH stays put. Then RH stays put and LH down and octave. Then both moved. Rationale: gets you away from using solely hand memory. I've found it very useful for counterpoint, JSB in particular.
2. Reverse the hands. RH plays LH where LH is supposed to be, LH similarly for RH. Very humbling; you find out fast how good your non-physical memory is. Rationale: as for #1.
3. Don't just start memorizing with m.1 and go straight through. Take a chunk from the very end, the middle, and close to the start. Helps you to be able to play from any point in the piece.
4. I sometimes find that memorizing a section HS first helps a great deal -- but be warned that once you start putting them together you'll think you've been wasting your time.
5. As you're memorizing, every so often put some stress on it. Play on a different piano. Play at odd times: when you start your practice, pre-warmup. First thing in the morning. I tried it once right after a long run and it was...humbling. First thing when you walk into your teacher's studio (I have to travel two hours, so this puts on all sorts of extra stressed).
6. Different techniques for different kinds of pieces. I find I can memorize pieces with lots of chords very easily, almost like "when I can play it I've memorized it." So I might spend more time working on starting at odd places in a measure. With counterpoint, I do this too, but probably more time on # 1 & 2 (above).
7. Many recommend doing in-depth analysis of a piece before memorizing. I've never found it helps me memorize, but many swear by it. I do work out various non-technical hooks, i.e. at one spot, LH descends a third, RH ascends a second. That is, hooks, whether formal or home-brewed, work for me on places I find hard.
8. Bernhard's dropped-notes technique. Like everything he's posted, tremendously useful. I use it for difficult passages, and for memorizing passages that I just can't "get".
9. Double-notes (I got this from a published interview with Mark Westcott, who uses it for scales. Play the first two notes together, then hold the second note and play the third, and so forth. Good both for hard passages and memorizing.
10. Triple notes. Play the first note with a strong accent, then the next two w/o. Then the next note with its two following notes.
I don't use all of these all the time with the exceptions of #3 & 5. Some may work for you, some may never work, some may guide you to something which does work.