If thin lips are what's stressing you, trace a line just outside your normal lip line with a liner matching your lipstick. You are faking a line, so don't overdo it! Fill in with the lip liner and add gloss to the center of the lips to draw light and make lips appear fuller. Play up eyes and use a more neutral shade on lips.

If shapeless lips are getting you down, trace the "V" in the center of the top lip a little heavier when lining to emphasize it, and then apply two coats of lipstick for durability.
If you are born with full lips, then flaunt it! A lipstain is gorgeous in all shades and lasts all day. Full lips are what we all want so use lipgloss for a natural look or leave it when using a deeper hue.... When applying makeup, feel free to down play eyes to draw attention to the beauty of your pout!
Just remember, if you feel the need to go get a filler, please don't go overboard and please be educated on the procedure! Or.....