There have been alot of great posts on here - thank goodness I made it to the last page!
I am on the design team of a Canadian online store
www.stampandscrapcanada.com They do not make stamps but sell products from many different companies and I love virtually everything they carry. So, suggestion #1 would be to try companies other than those who manufacture stamps! Online stores that simply sell other manufacturer's products need DT members too.
Suggestion #2 This company does not require alot of time. They send a 'care package' with various items in it. I can request certain items (though I don't necessarily get them) and also let them know if there are products I already have or do not want to get. Then I am expected to make 5-6 items - scrap book page, card, ATC or other items - from the supplies, I can add other items they don't sell as long as I keep it to a minimum, and also add any items I own that they sell other than what it in the package. So I only have to make a few items every couple of months. Perfect for me. So if you have limited time try a company like this.
Suggestion #3 Write good instructions (if this is required) Be sure to write them very clearly! There is nothing worse than reading instructions that don't make sense, miss steps or use unusual supplies without explaining them.
Suggestion #4 Make interesting cards. There are so many cool card instructions on here that are other than basic side or top fold card. Use them! This can help you get on the design team by getting you noticed. in addition, there will certainly be readers on the company's website who do not know about SCS and will never have seen such interesting cards. It will bring readers back over and over to see such interesting creations.
Suggestion #5 Make projects other than cards! Try home dec or altered items!
Suggestion #6 Be polite and professional. If you do not get the job, write a thank you note and ask that you be kept on file for next time.
I think that some of the other suggestions given were great: submit to magazines, have a blog (that is what I referred the above company to when they asked me to submit work-you don't have to add to it everyday, use it like a gallery!), photograph well, be sure you understand the requirements of the job before accepting and be persistent. Make sure your style meshes with the company's images or products. Ask a friend to critique your work, perhaps you need help with something simple like colour theory or proportion.
Hope this helps!