I recently solved a whole bunch of these.
- Old IPhO problems (first ~18 olympiads--at that time they were only amongst the Soviet countries). These aren't that hard. I have a RAR file of these;
I'll upload it tomorrow or something and paste a link.
- I.E. Irodov-Problems in general physics--these are fun at first, but later you'll realise that they are more calculus-based than physics based. You may have already solved these. Note that much of the book is out of IPhO syllabus, especially in electrodynamics. That shouldn't stop you, but since you may not have much time, then you should choose your topics carefully (the formula list at the beginning of each section gives you an idea of in-or-out of syllabus)
- SS Krotov- Much more fundamentals based than Irodov, and harder as well. I personally haven't solved much (don't have the book--don't have time); but whatever I have solved has been fun. But this is even more out of syllabus.
- Use Resnick for strengthening your concepts. Yes, you probably knew that.
- Take a look at past year IPhO problems(listed on the site). These aren't too hard either, though I believe that the time factor may change stuff.
- Most probably your country will hold a camp to train all candidates for this, but just in case: try to read up on experimental physics. Easiest thing to do would be to look at the past years IPhO experimental problems.
- The 2012 olympiad is being hosted in Estonia. Check out their Physics cup. When I last saw these problems, I really liked them...
- If you are trying for IPhO 2012, at this point your country will probably provide a camp(once you pass a qualifying exam). There they will provide you with books and stuff to solve, so you can just sit back and let them do the choosing :)
Just out of interest, what level have you reached?