I am an IUI success story! Got pregnant on the second round of IUI, first round using clomid. My FS was scannig me also to monitor and measure follicles. I was also using a CBFM to moitor hormone levels. It is all a bit of guess work to be honest, when is the best timing... For me, when she thought the follicles were about to release (I was ov'ing on my own anyway) we did the procedure (OH goes in 2 hours earlier to give sample, which is then washed).
The difference between my first (unsuccessful) and 2nd (successful) cycles was
1. I was taking clomid which menat that I had 3 x mature follicles which obviously increased my chances.
2. she used a trigger shot, althoug unlike most people I had this at he same time as IUI rather than 24-36 hours prior and
3. we BD'd that night too just to make sure!
Good luck - i hope it's successful for you too.