Here's an easy recipe that I've seen used in a number of casual riding situations with anonymous cyclists:
- You should take the first pull, so pass the rider on the left. Make sure you're going slow enough that she can actually hitch a ride without a massive sprint.
- When your rear wheel is passing her front wheel (ie. about the time she can see you without having to turn her head), pat your upper butt/hip a couple of times on her side. This should be a friendly "come here" gesture.
- When it's safe to do so, pull in front of her. You may also consider slowing down slightly to further communicate that you aren't interested in blowing by her. (But don't slow down so much that she'll just have to pass you again!)
- Once it looks like she's on, slowly and evenly accelerate up to your desired pace.
Keep in mind that some people aren't comfortable drafting and that others may not even understand the concept, so of course this won't always work.