Usual rules apply, just take all the tags off and dispose of receipts. I appreciate this might not be possible with items where you might take advantage of a worldwide warranty.
If you're planning to buy an iPad take your UK charger with you, put the iPad in your hand luggage (safer anyway) and the box in your luggage.
I've never let the tiny chance put me off, I'm ashamed to say I've spent vast amounts of my disposable income over the years in the US and never had a problem. I'm going to Florida this summer as well and plan to do more shopping, if I get caught on the way back so be it, I'm still winning overall!
Even if you are stopped, what's the chance that they correctly identify every piece of US bought shopping?