To complement Pixel's comment, choose your font carefully. Make sure it matches the "theme".
See attached picture: the words in the left column don't match the font, words on the right do. A "blacksmith" is usually associated with tough, hard work, a script font doesn't feel right. If the same font is picked that seemed right for "Blacksmith" is applied to "Frankenstein", a gothic or horror font might work better. Same thing for a word like "Spaceman" and "Fantasy".
Note that this is only an example, there might be better fonts out there that suits your need (and taste).
Colours is another thing that can make or break a graphic. Usually you are looking for contrasting colors, this prevents the image from becoming bland or dull. However, don't overdo it: don't use too many different colors, it makes the image distracting. Usually one base colour and two extra colours do well on a cab.
Next, stay in theme for the whole cabinet. If you are using straight lines in your sideart, do so in the marquee and CP. If you plan to use character art, make sure those are in the same styles (don't mix anime characters with marvel comic style characters).
Try to make the artwork work as one whole image. Many cabinets feature "collage" style artwork. Most of those cabinets look like (artwise) like a lunchbox with randomly pasted stickers on them. If you want a stylish cab, pick the character you like the most or pick a bunch of characters within the same style and make a design with your selection. The trick here is to keep it clean and not overcrowd.
IMHO, the coolest cabs feature their own set of artwork. This gives the cabinet the feel that it could be a cab that <insert favorite manufactorer here> could have made. Better to have a cab look like a specialist than a jack-of-all-trades-master-of-none.
note that a lot boils down on personal taste wheter you think a cab has fabulous artwork