1. Self-discipline (try to fly like an instructor or examiner is watching and critising every move you make) - if you keep flying during hour building in the same way as you were taught during PPL, you should be fine. Additional experience will bring finesse and self-confidence required for CPL.
2. Navigation: maintain good DR skills. Having said that, try to learn how to intercept and track radials/QDM/QDR, so that you are proficient in it when you start IR (you can do it on a Flight Simulator on your PC - it's cheaper and more efficient learning if you start by using autopilot and just turning the heading to see where the needles go). Learn how to use GPS (if it is installed in the aircraft you fly with) thoroughly - learn how to calculate winds, estimates, how to enter entire flight plan, etc. - not just the direct to, which 95% of PPLs do.
3. Instrument work: once you're good with intercepting radials, you can try flying an ILS - but ensure that you have someone, who will keep lookout during the time you look at the instruments (at least PPL, but I'd prefer an IR holder). Once you feel comfortable flying an ILS, you can try a non-precision approach, such as VOR or NDB.
4. Short field: try to "force" yourself to fly to shorter strips - your goal should be that you are able to land or takeoff with distance not longer than 50m than published data in the AFM/POH offers. Start with simulating this on longer runways and once you see that it's working quite OK, start flying to more and more short strips - although I wouldn't go below 700-800m at this stage (for a regular spamcan).
5. Study Air Law and EU OPS. As a holder of Commercial Pilot License you have a privilege to fly as commander in single-pilot commercial air transport. I know it's boring for the most part, but you should look at least what you can and cannot do commercialy with SEP/MEP, how to see if you have adequate performance for commercial operations, etc. I doubt anyone will ask you about Class A performance on CPL exam - although it wouldn't hurt to know if you're planning to fly something bigger in the future.
6. Have fun, go some places, abroad, new airports! Don't limit yourself to 1h radius around the airport. Try flying to some airport that is just at your maximum range (with all reserves included) - you'll see that you have to do much more fuel planning and monitoring this way than departing for an 1h trip with full tanks.