Healthy Ergonomic Tips for Bicyclists:
1. Proper frame size can be pretty accurately & simply gauged by straddling the upright bike with both feet on the ground. How much space is there between the top tube and your crotch? One inch? Less!? That bike's too big! Two to four inches clearance is generally what you want, though there can be some variation with different types of bikes and frames. Well-fitting Road bikes will definitely be at closer to two inches, while hybrids & mountain bikes will often fit well with three, four and even five inches clearance.
2. Seat height should be high enough so that while pedaling, your legs are almost fully extended at the bottom of your pedaling motion. If you're no getting this much extension, your knees will work alot harder than they need to. If you're locking your knees or find yourself sorta rocking back and forth when you pedal, your seats too high, also no good. (You may even decide at some point to try slightly longer or shorter crankarms, as you fine tune your comfort and performance needs.)
3. Your handlebar height/stem length should be to where your back is at about 45 degrees to the toptube with your arms relaxed-not quite fully extended, while riding. Your seat can be adjusted toward the bike's front or rear to help dial in your reach.
4. If you bike needs repairs, get 'em done. Just like how over time your bike's parts will wear faster if they aren't properly adjusted, you can do real cumulative damage to your body as well. For instance, if your bottom bracket is loose or a crankarm is bent, the slight irregular variation in your pedaling motion can horribly trouble your knees.
The most important thing is just listen to your body; and don't let price or time or anything else stop you from getting your bike fit perfect for you. And don't forget your helmet and lights!