I was told to plan difficult dead reckoning routes, plan to go through tricky airspace or tricky routes that you need to dog leg around danger areas etc - all in all just get your dead reckoning nav right up to scratch for the CPL. Get your 300nm route done, and choose an actual long route to do, not a load of local beacon hopping or something. Like bluerobin said also, do some flight plans, do some abroad trips and get used to crossing FIRs and difficult airspace etc. Lots of practice at MATZ penetrations is helpful too.
Another good idea if u can get it is fly with some of your instructors on a social level - if they are bored just ask if they wanna go flying and get them lunch or something in return for some free instruction to get you out of any bad habits and it keeps u learning, keeps u making friends and u never know who might get u a job in the future...
One of the biggest nags im getting from my CPL instructor at the mo which is a real point scorer with the examinier (and a possible fail if u dont do it enough) is LOOKOUT LOOKOUT LOOKOUT!
I was lucky enough to do alot of social flying with my instructors including some flying with the old IR instructor at my club - this was really useful as i hadnt done much instrument flying at all so was good for him to show me VOR tracking and intercepts and NDB tracking etc etc - this is really helpful if u havent done any instrument flying. I kept being recommended to do an IMC too as part of my hour building, as you have to do 10hours instrument flying as part of the CPL course, the IMC is a great intro to that and the IR - worth considering and its only 15hrs odd. I didnt do the IMC and i really wish i had to be honest.
Ive just finished my grounschool and im half way through the CPL. I did all that i just wrote above (minus the IMC) and its really helped...