Rule #1: Learn fancy JCS tricks with your own tileset such as palette edits that are such that your lives, health, score, and ammo do not show up.
Rule #2: Use lots of text. They will be so distracted with reading they will not notice your level is so bad.
Rule #3: Eyecandy to the max. Cover up everything! Make sure they can't see a damn thing, but they'll go "Oooooh..." and again forget your level is so bad.

Follow these rules, young Padawan, and you shall have yourself a decent level.

I don't actually think these rules are good. Don't follow them please.