1.The interrail pass gives loads of discounts.
Have a look and see what might be useful to you. Many other cities have underground railways but I don't think the pass is valid on them although a single trip in Paris, for example, is much cheaper than London: London is 4 quid a go and Paris is 1.2 Euros for a single trip. Prague is also cheap at around 70 pence a ticket.
2. Present your pass when you book a journey. You'll then be given the discount (or not!)
3. I've just come back from Poland, Czech Republic and Switzerland. You'll need the local currency and you get a better rate if you take it with you. In Switzerland I used Euros but ONLY in the railway station.
4. On any journeys keep a close watch on your bags. Some countries are more risky than others. I locked my stuff to the luggage rack several times. All baggage will be targeted by thieves anyway so make it difficult for them if you can. Just before departure there are people who roam the trains looking for unattended bags to jump off with. I've met several unfortunates who had been robbed. Just be aware and vigilant; it's the best you can do.
5. Put your pass in a waterproof plastic bag: the ones that you get your currency in are perfect. After a month my pass is now pretty tatty even though it was always protected. And if your pass does get wet, it will disintegrate.
Hope this feedback is helpful.
Have a most excellent trip.