IMHO, if you're really set on improving your resume, a service is worth it. Use one for the cheapest option possible and run it a few times on your resume -- you'll see the patterns and things it looks for.
In general, it would be things like:
* Avoid repetitive action words - "Built systems, built software, built infrastructure..."
* Add numbers/metrics where you can: "Improved site performance by 20%+"
* Line items should not be over 2 lines
Aside from that, choose a _standard_ resume format/layout. Don't get fancy with columns, colors, etc.
