Yep totally agree, you should only play pieces you love, life's too short to just play pieces you like.
There's some great suggestions in previous replies to this question. For what it's worth here's my method for memorising a piece:
1. Have it clear in my head before I start that I'm going to memorise, this means I (both consciously and unconsciously) start looking for patterns, scales, arpeggios, and key centres right from the start.
2. Just play the piece with the music for a while (can be a couple of months, especially with a beast like the Waldstein)
3. Divide the piece up into its natural parts (like mentioned earlier) With the Waldstein it's first mvt sonata form so you need to know what's happening at the start of the exposition, development and, recapitulation. Even in the exposition, you should be able to picture the first chord (tonic of C major) of the 1st subject, then the first chord (E major) of the 2nd subject. It might be worth having a photocopied version of the score you can write harmonies and sections on, then once you've written the main sections and harmonies on this, close your eyes and go through them away from the piano.
4. Divide the piece up further so you can start from each page. Go through the starting points in your mind away from the piano
5. Divide the piece up again so you can start from each line by memory. Again, go through these points away from the piano.
(this last one seems extreme, and it kind of is, but it's SO useful to have a piece this well memorised for performance. I usually play the piece once a day backwards at this point - not literally backwards, but starting from the last line and playing till the end, then the second last line and playing to the start of the last line etc. Some of the start points will feel unmusical, but that's ok, you're preparing your mind for the performance)
6. Play it without the music, but super slow. So slow that a person who knows the piece and is wandering past might not even recognise it. This is hard, and takes concentration of steel, but is gold.
One last thing, and this kind of goes back to point 3, but how I would attack a piece like the Waldstein to memorise is with the harmony and patterns would be something like this:
Starts with tonic C major chord, moves via 3rd inv D7 chord to 1st inv G chord. Then same pattern starting from Bb major (wow, awesome harmony change btw) moving via 3rd inv C7 to 1st inv F major, then 1st inv f minor etc etc. On the 2nd page you have the left hand doing the B major arpeggios while the right does that cool run (worth memorising separately) culminating in the B major to E minor arpeggios. Then the transition section moving into E major for the 2nd subject
that's a very rough version but you get the idea.
Hope that's a help, man I love this piece, all 3 movements are awesome. And if you stick at it it will be worth it as you can get so much out of it
good luck!
Dan