It may not be the best way but the following has worked for me - after trying a few suggestions;
> edit > preferences and select the tab 'IMPORT SETTINGS' and choose MP3 Encoder fom the drop down list (where it say's 'import using').
Then select all your songs (song list and not album list), right click and copy as MP3 file (may take a small while depending how many congs you have so make a brew)!
Create a back up disk from your itunes - > File > Library and select 'Back-up to disk'.
Close itunes and from there it's just a case of dragging and dropping the files from your back-up disk to your sansa clip - The MP4 files won't transfer to your clip anyway but the MP3 do.
If you want and have the time you can always then delete your MP4 files ;
Right click on the song - select 'get info' tab and at the bottom of the summary tab it will say which file type it is. Obviously keep the MP3.
The bonus is you have a back up disk already created for your sansa too.
Hope this helps!