Most customs delays are self-inflicted, so a bit of preparation goes a long way. Keep receipts for anything valuable in your hand luggage, pack food items and new purchases near the top of your bag so they're easy to show, and leave original boxes at home for your own used electronics so they don't look freshly bought. If you carry regular medication, keep it in original packaging with the prescription.
Know your destination's duty-free allowance before you land and honestly tally what you're carrying. If you're over, or carrying anything in a gray area, go to the red channel voluntarily. Officers deal generously with people who self-declare and harshly with people who gamble.
When questioned, answer directly and briefly. Say what you have, where you're coming from, and why you're visiting, without volunteering long stories or getting defensive; nervous over-explaining draws more attention than the items do. Dress normally, don't film officers, and build twenty spare minutes into any onward connection just in case.