Paisley - I learned the hard way not to book flights until I got my positive OPK. All of my cycles were regular, ovulating on either CD15 or 16 until the cycle where I booked in advance a four-day trip to Copenhagen across my CD14–17, then I ovulated a week late 
so I ended up having a city break with no fertility treatment and then going back a week later for IUI. It was a lovely example of Sod's Law!
I suppose it depends on availability of flights from Belgium. From my local airport (Luton) I had six flights a day to choose from all run by budget airlines, so even booking the day of travel or the day before travel my flights generally cost less than £100 return each time.
So my approach has generally been to wait until I get a positive OPK then drop everything to book flights.
For hotels, I abused Booking.com and for each month booked hotel rooms a couple of months in advance with free cancellation across my predicted CD14–17 and then cancelled each night once I knew I wouldn't need it. I actually feel quite bad about this; the hotels must have hated me for it! If you take this approach, you need to book each night as a separate booking so that you can cancel individual nights.
Which clinic are you using? If its anywhere near where my clinic is, I can recommend hotels, restaurants etc. 