July and August are the most expensive months. June and September are back to normal pricing. So maybe try to fly less in July and August, how long will you be staying?
As for when to book, generally I book about 2 months in advance for budget carriers, sometimes you can get away with a few weeks, but again in July and Augst the latter is going to be difficult.
Do you have luggage with you that needs to be checked? Then even flying around on budget airlines won't be very cheap at all.
I'd suggest to fly only when necessary, and take the train or bus between cities that are closer to each other. For example, you could go from london to paris by train, then on to brussels and amsterdam by train, then amsterdam to berlin by train etc. The bus is often dirt cheap, but most people find trains more comfortable than busses.