We've all done it - fairly innocuous errors to catastrophic Nav faux pas. The only solution is simply practice and learning from experience so keep it up!
I'd echo the "keep it simple" philosophy. I've had many gadgets recommended to me over the years and seen pilots with gigantic flying bags crammed full of all-sorts. All these do is drain the flying fund, annoy the missus with "What on earth are you spending money on for your flying now?", increase your take-off weight and make things more complex. When it comes down to it DR and UnP Diversions are a bit of fairly simple arithmetic. No calculators or fancy apparantus needed.
Nowadays my flying bag consists of one of those little headset cases carrying my headset, kneeboard, checklist and chart. That's all. I like it that way
