In the past I've included the number of years I've been using a skill, but that's still not the best measure of what skills you have.
My suggestion would be to only list the things you are 'good' at, and if you've played around with something but not really used it, don't put it on there. I'm sure tons of people put every possible related piece of software they have used like:
At that point the lesser skills like 'using a computer' start to outnumber the key skills, like technical writing. I'm not sure if this is good or bad, but I know it would make the valuable skills harder to see if they get lost in less-good skills.
A résumé isn't supposed to list every achievement, skill, and piece of experience you have, that's what a Curriculum Vitae is, a résumé is supposed to be a 1-page summary (the word résumé literally means 'summary' in french) of the most relevant skills, experience, and education from your entire life's work (your CV). If it's not directly relevant to the job you're looking for, leave it off your summary :D