All kidding aside, my first upgrade is almost always a 6.03 tightbore barrel. I'm partial to the Prometheus brand but Mad Bull barrels are also good.
Aesthetics have never been my strong suit (insert duct tape joke of your choice), obviously. A custom paint job works wonders on almost any gun. There are plenty of styles and techniques. You can find information online or in magazines.
So Go also offers gun painting as one of its many customizing services.
Different grips, stocks and add-ons (from the practical to the tacticool) also help your gun stand out from the crowd.
EDIT: I also like to add a red dot sight to my AEG replica long arms. This is not for style but for a practical purpose. Properly dialed-in, the sight lets me know where the bb is most likely to hit. I guess that counts as customizing.
Sometimes this requires adding a sight rail or an elevated sight rail before I can attach the red dot sight. I guess those also qualify as customizing pieces.
Thank you, Metal. I appreciate the compliment.