If you have an ant problem inside your house (or any kind of pest problem) and all else fails, you should get professional help and call an exterminator. However, it is my experience that with a little knowledge about ant behavior and a little bit of patience, you can take care of business yourself and not have to spend a lot of extra money.
Spring and summer are generally the times when ants invade your home. The main reason ants come into your home is because they are searching for food. You'll often see them in the kitchen/dining room area.
Typically, the ants that you see are only the tip of the iceberg. So if you just see one or two at a time, don't assume that there are only a few. Most likely, these are scout ants that are searching for food and will signal more ants once a good location is found. Plus, the ants are most likely nesting in your walls or other hidden parts of your home, or outside.
Killing the ants as they appear is not the best solution. It's like cutting a weed instead of pulling it out at the roots. Not only do you not get at the heart of the problem by killing ants one by one as you see them, but you also can leave a dead ant scent that signals other ants to retrieve the body (at least this is what I read, as I have not observed ants carting away their fellow ant corpses). Ants also leave a scent trail wherever they go, so if you see one ant, chances are another one is on its way down a similar path.
The key to getting rid of ants entirely is killing the colony at its core. To do this, you need to poison the ants that visit your home but keep them alive long enough to bring the poison back into the ant nest to kill the rest of the ants and, most importantly, the queen.
Since ants are coming into your home looking for food, give them what they're looking for. I've tried a number of products, including those little plastic ant bait stations, and all seem to have some success. But I think the simplest and cheapest way to go is to buy the gel (Combat brand works well) and squeeze small globs in the corners of the room and by cracks in the wall or cabinets or generally any places where you think you might have ants entering the home (or have seen ant activity).
When I first discovered the ants, I watched them for awhile (it does take some self-control not to kill them immediately, particularly when they are in your kitchen), and followed their activity. You can pretty easily find where they're coming from as they are usually either entering your home or leaving it and they take pretty direct routes.
Anyway, as soon as I squeezed the gel globs on to the ground, within about 20 minutes, I had swarms of ants around the gel globs closest to their entry/exit point in the wall. At the time, I had no idea where the ants were entering the home, but by seeing which globs got the most attention from the ants, I was able to find the hole.
I had to reapply the gel a few times to the floor where the ants were eating, as they ate it pretty quickly. I wanted to make sure I gave them as much gel as they wanted, so that:
(1) they would not wander off into my home looking for more food, and
(2) they would take the food back with them (I think they bring it back and regurgitate it) and spread it to the others.
Again, this took willpower not to kill them all as it is not a pretty sight seeing large groups of ants snacking away in the corner of your living room (see the picture above).
The gel I bought from Home Depot only cost about $4 per tube and I ended up only needing two tubes Spending less than $10 to rid my home of ants is a pretty good value.
The ants were gone within 12 hours and I plugged the hole that they were coming through. No more ants for the rest of the spring and summer. I will use this technique again the next time I have an ant problem.
If you have an ant problem inside your house (or any kind of pest problem) and all else fails, you should get professional help and call an exterminator. However, it is my experience that with a little knowledge about ant behavior and a little bit of patience, you can take care of business yourself and not have to spend a lot of extra money.