If I could offer up another piece of advice, try using a better grade of masking tape for the actual paint demarcation lines. If you lay out your design with high quality tape (Tamiya modeling tape is my favourite) and then use regular, inexpesive masking tape to cover the rest of the model from overspray, you'll find that the modeling tape will you a cleaner, tighter paint edge. Modeling tape is thinner and more flexible than painter's masking tape and will follow surface irregularities much more closely.
Check out the "go faster" stripes I applied to a Viper MK2 model I built for an example of the technique:

I also agree with Quaralane, a base coat of paint is an essential foundation to start the rest of the paint scheme. Even if you just apply a thin layer of grey sandable primer from a rattle can, you'll find that the subsequent layers of paint will have something to grip to and give you a more even finish.
Keep it up and don't get discouraged. This forum is a great resource and it's full of people willing to offer well meaning, constructive advice.
Cheers.