The best way is to convert your footage to 24p before you start editing; but it depends how much footage you have. It is extremely time intensive.
You can also do it when you finish your cut. In either case you use Compressor — since you are using FCP5 and Motion2, you must also have Compressor 2.
Compressor 2 can do very high quality frame rate conversions and de-interlacing, using the same optical flow technology that is built in to Shake. The results are better than any plugins currently available for FCP, better than DVFilm and better than Magic Bullet, but it is slow. Basically you create a preset in Compressor that will take your program and convert 29.97i to 23.976p. Run some tests on material that has cuts and dissolves in it though, to see how they turn out — I’ve only tested it on single shots; but I promise if you do it at the highest quality settings, you will be stunned by the results.
Look around on the FCP board for other techniques vis a vis Compressor. It’s also a very powerful way to speed-convert clips.