You have to show proof of sufficient income. This may or may not be $1500 USD per month, but that's a good place to start. You have to show this for the last three months, not for 12 months. So you have many different options to accomplish this. First, if you are receiving $1500/month, just submit the last three months of statements after having deposited this money in your checking account. This will do the trick. Mind you, you don't have to show that you are spending $1500/month, only that you have $1500, preferably more, in your checking account for the last three months before applying. Optionally, you don't have to show that you have $1500 or more in your checking account at the end of the month if your statements show that have been depositing at least that amount each month. So you can work it either way. These statements you submit will usually start four months before you apply, since they come out end of month.
Another option is to show that you have enough capital to live for another year in Mexico. We do this by simply having our broker at Multiva write a letter to immigration stating that we have more than X pesos in our Mexican investment account. This probably doesn't pertain to your situation.
Finally, as a last resort, one can "churn" a checking account to make it look like you have been depositing and spending more than the minimum each month. This takes a little doing and some help from family or friends in the US. Without going into this much more, you are basically making payments from your checking account, then redepositing that money back into your account sufficient to show the required amount of monthly deposits. It doesn't sound like you would need to consider this either.
The main point is that it is only the last three months each year when you apply or renew that you need to present these documents/fulfill on paper these requirements to immigration. From what you have said, in your case it shouldn't be difficult.
And once again, you can avoid the hassle of applying for work papers by just ignoring the whole issue as long as your clients are outside of Mexico and they pay you outside of Mexico. Technically this may not be totally kosher, but practically there is no way for anyone to find out less your bragging about it.