In my opinion it’s best not to book too far in advance, it takes the spontaneity out of it. July and August are the busiest months, so you should be okay. There are plenty of good websites, so just book one or two days beforehand (maybe three or four if it’s a city and you’re heading there on a weekend).
 
There are, however, exceptions to this rule. If you are going anywhere where there’s a festival going on, or about to begin, get in ASAP as the entire town will book out. Make sure you check your guide book. Also, if you’re arriving somewhere late at night, it makes life easier to know you’ve got somewhere to stay.
 
Be aware as well that some websites will tell you there’s no availability, but the website is only allocated a certain amount of bookings, so often there is actually plenty of room, just call and double check.