Develop characters and situations that will allow you many avenues for humor in the future. Very few aspiring cartoonists pay enough attention to their lettering. The words need to be lettered neatly enough, and large enough, that readers can read them without difficulty. Have your local copy shop reduce a few of your cartoons to printed size to see if your lettering is still legible when reduced. There shouldn't be too much writing either.
People prefer reading shorter, quicker-paced comics.
Many aspiring cartoonists don't use waterproof drawing ink to finish their drawings. Pencils, ballpoint pens, and most felt-tip pens don't reproduce well enough for syndication. Finally, many aspiring cartoonists develop comics that are too similar to already successful strips. Newspaper editors aren't going to duplicate a comic that they already print. Yes, King Features is made up of several previously independent syndicates.