It would seem that everything is straightforward if the US citizen sponsor lives and works in the United States. However, a lot of citizens are married to aliens and live abroad. How do they go about filing documents like I-130, I-864, and I-485? Actually, the process is not very different and they are as eligible as a citizen who lives in the US. There may be just additional paperwork or they may have to wait to take certain steps before filing.
Domicile in the United States: In order to be able to sponsor, you should be domiciled in the US. Domicile in the US can typically be established if you vote here, own property, pay state/local taxes, have bank or investment accounts, or simply have a permanent address here (thus, it is fair to use a family member's address if you receive your mail there). If none of this applies to you, do not lose hope. Chances are that the reason you want to sponsor your spouse for permanent resident status is because you want to move back home. So, before you do your paperwork, you may have to take other steps a little bit ahead of time than other people. These actions can include buying a home or signing a lease to rent one, some proof of registering your kids to attend school in the town where you intend to live, or a job offer. If you are unable to accomplish actions like these, you must simply wait to first relocate to the US and then do the paperwork.
Have you kept the Tax Man happy? You might already know that all US citizens are required to file a tax return regardless of where they live. If you have been meticulous about your taxes, you are in good shape. That information will be needed to complete the paperwork. If not, make sure that you first make IRS happy by filing whatever returns you need to or get exemptions if that is situation in your case.