You need to ask yourself if you think you can do better. When I took in Sept, I knew my score would be way lower than my pratice tests and target score just because I guessed a lot on games which is usually my strong suit, and I didn't feel good about RC. I figured my nerves got the best of me. I took time off, only practiced about 3 tests worth before Dec, and I think I KILLED this test. For me, cancelling was the best thing I could have done; if I had kept my score it would have been so much lower than this time around it would have either a) watered down my average or b) would have required and addendum.
If you don't think you are going to improve, then I might not do you any good.
In the long run, it is probablly better to ensure that you put your best foot forward when applying, even if it means waiting a year. If you are fresh out of undergrad, the extra year could be helpful provided you do something worthwhile. If you are older (like me), it's just another year.
As far as what I did to help myself do better:
1) I stayed away from this board. I think reading post after post of people freaking out made me nervous.
2) I had already developed my skills, so I only did a few sections a week starting about 2 or 3 weeks prior to the test to refresh. I think this prevented me from getting burnt out.
3) When I timed myself, I used 32 miuntes as opposed to 35, mainly because I ran out of time on the Sept test too much. This improved my timing and accounted for any unsettle nerves I might have had on test day. I finshed each section with about a min or more to spare.