I'm not the most expirenced bloke around only being half way thru my CPL, but one thing I found useful when doing my flight plan was to write small notes to myself next to or under each leg of the plan.
For example you may have Centre/Radar frequencys, a small reminder to contact ATC at a given distance, a note to yourself that you will need to commence a descent at a given point etc...
I found that forced me to think ahead while flying and plan for what I'd be doing next. It wasn't so much of an issue on later Navs, but it did help me for the first few.
Oh, and cockpit preparation will help too. Pencils, ruler, protractor and the 'ol wizz wheel need to be in easy reach, maps need to be properly folded and in the order you plan on using them .. all that sorta jazz.
Others will have more advice I'm sure.
Cheers,
AB