1. Check the 'tude at the door
Yeah its common to be stressed when out on the town if you're running late or have ran into problems at your last stop or maybe its even something that's bigger than that. whatever has you bugging out is no reason to give the employees of the business you just entered a hard time. If you aren't good at controlling your rage at least apologize to whomever you inevitably snap at for no reason. If you can leave your problems outside and you will 100% definitely have a better experience.
2. Is it just the rules
A lot of businesses dealing with customer service have a set of rules the employees have to follow that are bound to be unpopular with the customer. Whether its the cashier at the supermarket, the waitress at the restaurant, or the drive through worker at the fast food joint, they all have protocol to follow. If one of these guidelines upset your plans its important to remember that there's a 99.99999% chance the person your talking is not responsible for creating them. If it is a serious issue to you politely ask to speak with the authority figure without being an asshat to the person your dealing with. The manager/owner may not have had a say in the rules either if its a chain, but they get payed to have you scream at them, unlike that minimum wage worker who broke the bad news to you. Even better: accept the rules for what they are and drop the issue, especially if its something that was probably printed on your receipt*cough*merchandisereturndeadlines*cough*
3. react appropriately!
People aren't perfect, and something is bound to go wrong eventually. maybe a waitress screws up your order, or a cashier accidentally scans an item twice. whatever the mistake just remember to react appropriately. Its ok to be mad, as they DID screw up, but most of the time they will immediately apologize for the mistake and do their best to fix it ASAP! In the end you will probably only be delayed by 10 minutes or so, not have your house burned down, so tame it down to a reasonable level! also note that rude customers embarrass themselves in front of the rest of the store...
BOOM! 3 easy steps to improve the experience for both you and the employee(s). be pleasant and they'll return the favor. be a grouch and they are gonna end up just as pissed as you.