Effect Vs Affect

[Mistoffelees]

My understanding of grammar isn't too bad but this is one that I always struggle with. I know the general rule and have read any number of 'easy ways to remember' but can never seem to apply it correctly. Can someone explain it in very easy to remember terms, preferably with a few examples.

Thanks all, I've re-read the answers and I think I've got it straight in my head now. Will probably re-refer to it over the next few months until I'm 100% confident