In 300 miles I would say you should have (a) lubed the chain several times and (b) replaced some brake pads (c) put a few drops of tri-flow lube on all pin joints in the derailers. There are online tutorials showing you how to do these things but it will take time and effort and you'll get grubby hands. Perhaps you're thinking "yuk" as you read this!! That might be the reason why you continue to take it to the LBS for a service. It's also the reason a lot of people blow all their money and never own a house. A bike is a good place to start learning to do things for yourself and save loads of cabbage. I feel sorry for you if, unlike myself, your Dad or Mom wasn't able to pass on these skills, or didn't bother. If by any chance you live near me, come around and I will help you get started - so long as you are prepared to get grubby hands.