Don't think HMRC are stupid. They have long memories and are being given more and more resources to track down individuals and companies who aren't complying with the law.
If you've moved home or you have a new business address make sure you tell HMRC of your new address. You may think you've shaken them off but the penalties are still mounting up in the background.
Keep proper records. Technology is making this so much simpler than a few years ago.
If HMRC write to you don't ignore it. Reply as soon as possible, even if it's only an acknowledgement.
Self assessment was introduced on 6 April 1996. It was heralded as making taxpayers responsible for calculating their own tax liability rather than expecting HMRC to do it on their behalf. Forget that. You're on your own. HMRC won't help you, you're in charge of your tax affairs.