DFW - IAH is usually not difficult, provided that you can be somewhat flexible with time of day.

DFW - EWR is more difficult, requiring you to be flexible as to departure DATE, not just departure TIME.

DFW - CLE is not difficult (but is this something you plan on doing?)

DAL (Love Field) - IAH on the ERJs, in my experience, is always the best bet!

Nonetheless, if you plug in any random city pairs from DFW (such as DFW - MIA, or DFW - LGW) you will often run into problems because the class of service available (even on reward tickets) between "shuttle" flights such as DFW and DAL to IAH are not in the same buckets as the long-haul flights you might be connecting to at IAH, such as nonstops to Europe or deep South America.

So, I find the easiest thing to do (and it almost always works) is to go ahead and book the IAH - XXX portion first, usually online. Then, I call the service center or web-help to get my DFW - IAH or DAL - IAH segments added to the IAH - XXX segment. Of course, you may luck out and find availability immediately, even with the connecting segments. But if the "easy" way doesn't work, I have found this method is usually very effective.