...when you're not the one pushing it!
My husband's flexible work schedule allows for us to occasionally run together with the double stroller while my oldest is at school. I've joked in the past about who gets to push the stroller since we are basically the same speed. Recently we've just been splitting it first/second half. It certainly takes practice to push the stroller for miles, but there are also some tips for the person running alongside.
1. Let the person pushing the stroller be the pacer. This is easy going uphill when it would obviously be very obnoxious to go ahead. It can be frustrating when the stroller speeds past you downhill, but it is very hard to go any slower on a steep hill so let them get a little money in the bank and you'll catch up easily when things level out.
2. Run on the left if you are on a path. This allows the stroller to stay right at a relatively constant pace. On the left, you can easily drop back if someone is coming from the other direction and then come back up again. If you are on the streets, do the same on the right (i.e. the traffic side).
3. Don't dive in to give the kids stuff or make adjustments that require stopping without consulting the "pusher" first.
4. Don't grab the handlebar to "help" push uphill without asking first.
5. Don't cut over curbs and corners that the stroller has to go around...and then stand there like, "Hey - what's taking YOU so long?"
Anything else that helps you?