For domestic travel there is really no need to look beyond online resources, which I'll summarize in a moment. For international, you'd do fine online, but there are travel agents who specialize in specific markets and can be of help.

I like to check as many resources as possible. If you get deeply into travel and FT, you'd find ITA matrix.itasoftware.com to be a powerful resource. They don't sell tickets, but that's why they can be objective. Best place to do research if you take the trouble to learn their synatx.

Next is Kayak.com - it looks at various portals for you. Sign up for the free membership; then it will remember your previous searches and also let you do flexible date search. If you take a liking to a specific portal (Orbitz.com, Travelocity.com, Expedia.com, etc), look at them too.

All this search will begin to point to a few airlines that seem to be most promising for your itinerary and dates. Check their websites too. Sometimes they have promotions not offered outside.

As I said, for international travel, I like to check with a few ethnic travel agencies (for example JTB for Japan, jtbusa.com/en). Every market has its specialists, you'd have to discover them depending on where you travel to. (However, tix sold by such agencies may or may not earn frequent flyer miles, so check before buying.)

Similarly, for travel within a foreign region, for example Europe or India, get to know a few good local agencies in addition to the big US based ones.

That's about it. I strongly urge you to choose one airline and concentrate as much of your flying as possible. When this is not convenient, choose a "partner" airline which will give you miles on your primary one. Similarly, get affiliated credit cards etc. All of these points add up and are useful.

Good luck.