How do you do it?
I've just been getting a new cheapo stamped offset serrated at smart and final, as the other one wears out. I usually just use it for bread. My main knife usually does tomatoes fine.
There are a lot of different styles of serration. Many of those with the "teeth" set very close together can be sharpened using any of a number of manual and electric "pull throughs."
Many wavy edged knives can be sharpened using a "slip stone" or "file" of one sort or another to sharpen inside the wavy bevels on one side of the knife. (Note: In sharpening jargon, any cylindrical, round, or half-round sharpener is a "file." Files don't have to be metal.) DMT makes small, diamond-impregnated files intended for "field sharpening" pocket knives, but which are very useful for sharpening wavy edged, kitchen knives.
I wouldn't waste a lot of time or money sharpening a cheap knife, since you get so much mileage out of them and they can be replaced so cheaply. But that's me.
If you want nicely sliced tomatoes, you ought to keep your fine edged knives well sharpened. Tomatoes should never be much of a challenge.