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To answer your second question first, yes, there are discounts for ISIC members and are quite useful sometimes. Check at the specific booking counter since some countries don't accept it (you'd think that all the countries have the same rules for Interrail pass but you'd be wrong).
And when it comes to prices, it is best to buy online, and the prices don't change that much during the year. If you're traveling in June or July, there will probably be some kind of a discount because of Euro 2012
in Poland.
I see you're from the UK, just so you know, your Interrail pass doesn't cover train travel in your home country (at least it doesn't in Croatia) so you have to get to mainland Europe first.
As I've seen from your other post, you will most likely travel solo which is nice but if you had at least one more travel mate, you would qualify for great discounts if you traveled on Thursdays or Fridays. For these discounts you usually have to get to the ticket counter and ask for the cheapest ride in a week. Those are almost always early morning trains. If it's more than two of you, the price goes down even more (it sometimes gets to 50-60% less than the original price).