Chapter 1 - Becoming a good Creator
Each editor has its own secrets, which are ready for you to explore. Many of the successful creators have been with Spore since the beginning two years ago and naturally have progressed so far to know their favorite editor inside out.
If you like a particular creation, don't hesitate to take it apart in the editor to see how it was done. In the beginning it can be hard to even retrace the steps but with enough trial and errors you'll eventually get the hang of it.
If possible, get the most out of your creations by reaching the complexity meter limit, carefully choose the textures and colors and don't be afraid to take your time. The more you spend in the editor the better you'll get.
Chapter 2 - Getting on the MPN
Being on the MPN is a great feeling, especially if you're new to the game or haven't been in the spotlight before, but remember that nobody will notice your work unless you make them aware of it.
Several times a week, go on the MPN and comment on (and uprate) anything you like. Don't ask to have your creations looked at, just leave a nice comment or a creative suggestion. Other players will enjoy that more than needy-sounding comments.
Chapter 3 - Increasing Subscribers
The same rules apply to the subscriptions as to being on the MPN, but additionally you can ensure someone coming to your page and buddying you for a few reasons.
You could either specialize in themed sets, specific artwork or make highly detailed props that other players could use or be exited about seeing in their game. With enough practice you should get together a nice portfolio that could attract a couple subscribers every week.
Chapter 4 - Treating other Creators
Spore is a game with an in-game networking system. Since Maxis and EA can't or won't censor your speech in comments you should watch your potty mouth. Too much foul language can get you reported enough to being banned.
If you want to be treated nicely you should do so, too, since what you give is what you get. And who
Chapter 5 - Dealing with Downraters
If another creator maliciously puts down your work and constantly harasses you, immediately report their nasty comments and then block them from commenting. It is best to ignore downraters as all they're looking for is a reaction. This also means actively going against one or more downraters.
If you keep giving them the attention they will have won long before your newest creation has a red face.
Have patience and keep creating, Rome wasn't built in a day either. :D