Post your receipts back to you, remove any tags from clothes. Basically take virtually nothing out in your suitcases when you go.
£145 is the level, though I've been back from US three times and not been stopped - tbf the excess tax you'll pay if you have to will not hit you that hard considering the decent exchange rate that we have in the UK now against the $.
If you are feeling particularly guilty as you come back through customs may well be worth going through the red channel.
Depends on what you are intending to bring back ofc. They're sure to be watching you through the two way mirrors anyway