Well I have tips to do it on the cheap. I know when I had an eurrail pass (same as interrail I think), there were only a certain number of days it could be used. So we would use it on really long distances. But if it was a short trip or we were in a less expensive country, we'd just get a cheap train or bus ticket and not waste one of our eurrail days. Also, we'd take some overnight trains because it would save the hostel cost for the night.
I'd advise to think beforehand rather accurately your schedule and plan (even though I know most people probably want to change location and determine their stay "by feeling"), but it helps, so you dont realise 2 days before the end that you were stuck in 2 cities the whole time. I'd also advise to emphasize on the big cities and capitals, since small towns and countryside may seem idyllic and all at the beginning, but reveal to be usually all the same and quite boring...