Sorry that wasn't clear...

ITASoftware makes the engines that some (many?) travel sites use. They do not sell tickets though.

So once you find it, you need to use another site. I usually go direct to the airline and have only done this with Delta.

If you're using Delta.com, you should do a multi-segment trip. That allows up to 6, if I recall correctly. You may be doing quite a bit more than that (I was). In that case, I found that entering pairs where I skipped the hub city worked well.

So, imagine that I was doing MSY>ATL>BWI>DTW>MKE>MSP. ATL, DTW, and MSP are hubs for Delta. I could just enter them individually but I'd have had no room for the return flight (which was MSP>MKE>ATL>MSY) so I needed to compress the flight some. I believe I entered it as follows:

MSY>BWI
BWI>MKE
MKE>MSP

There aren't that many (any?) MSY to BWI flights for segment 1 so it shows options for routing through Atlanta for segment 1. Viola, you've got more than one flight segment per segment entered into the multi-segment search at Delta. There are limits to this but you can probably get to 10-12 total segments this way. I assume the approach with other carriers would be similar.

I'd sure love anyone else that's done thus stuff to pipe in! I've played with this a fair bit but only really flow two segment runs. There's probably a lot I don't know.

Good luck!