I wouln't say I'm very wise (as per the thread title), but I have felt the pain.
My biggest mistake.. not concentrating 100% on the training. Instead I let some smallish business distractions get in the way and that lead to taking expensive steps backwards not forwards.
So far as the flying is concerned, everybody will have their own MUST DO tip. Mine is trim, trim, trim then relax, relax, relax. If you can do this, the a/c will carry on doing what it's supposed to if you have your head down looking at the checklist or plate a bit too long.
Best of luck.