Here is my tips for newbs who want to design but aren't real creative:
Stay away from free clip art, fonts already installed on your
computer and design everything with good vehicle outline templates.
Don't disproportionally scale fonts...if it needs to be skinny and tall, find the condensed version of the font!
Don't put artwork too close to the edge... when I make a wrap..I like to have several inches of wiggle room...this makes the install much easier.
When doing a wrap, avoid plain backgrounds. Check out the stuff at auroragraphics.com... You can save yourself plenty of time by just purchasing a cool background and modifying. This stuff is expensive enough that its not everywhere and therefore lame.
Do make all text and logos HIGH CONTRAST off the background if the vehicle is used for marketing a brand. If you can't read the brand name from 60' and 60mph then you might need to re think your design.
In the end, your client is always going to have you do things you don't like. Learn to warn them in advance so that when they hate it later you aren't feeling responsible.
Also, even the best designers have their bad days...so don't feel bad thinking everyone else out there is perfect... because they (we) certainly are not perfect. This industry is not a competition...its a business. If you are happy and the customer is happy... thats all that matters.