A 10-step guide that's easier than AA!!
Preliminary Note: if the foreign spouse [not the US Citizen spouse] has more than 40 quarters of earnings which were subject to US Social Security, no I-864 is needed. This is proven by use of form I-864W signed by the "foreign spouse" with proof of the Social Security earnings. This situation is more common than one thinks -- prior work may have been on prior non-immigrant visas [E, H, L's], a prior period of green card status, or even long term illegal residence in the US.
1. Ask yourself what evidence you have already that you can use and the next question you should ask yourself (out loud is always good) is how can we prove that we are serious about breaking ties to the UK.
2. Make sure the US spouse has filed a US tax return each year. Even if its nil.
3. If you are going to reside with an in-law, then you should look to formalise that relationship perhaps with a contract. Others will disagree with me, but when we applied, we had a letter from my in-laws saying we could have stayed with them, it was rejected on the basis that it was too informal. The CO said explicitly, unless you have a lease, you will not succeed. That is NOT to say that it is like that in each case, but that scenario is occurring more frequently.
4. Provide proof that you are house-hunting, job-hunting, enrolling kids in a local school, enrolling in local education programs yourself (if applicable), car ownership/title. All of this will help prove that you are active in establishing a life in the US. After all, we all need to live somewhere to live and earn income.
5. Going back to one, if you don't already have this, get your spouse to have a US bank account, US drivers license, proof of their US voting record. All of this will prove that they haven't given up their US life entirely.
6. Perhaps your spouse have been successful in finding a job, and may have a letter from the new employer stating that - that would be very useful. Failing that, evidence that they have been very active in trying to secure employment. Again, think of the basics, where is your US income coming from? If you are merely job hunting that is fine but prove that you are.
7. The big question these days is evidencing intention to leave the UK. Remember you only have to prove your intent sufficiently - you do not actually have had to left the UK.
Good ways of proving this is a letter from a estate agent proving the property where you live is for sale, or that your landlord has noted your intention to relinquish the lease you have. Perhaps you have given notice to your employer, that too would be useful.
Perhaps you have also relinquished financial affairs in the UK - sold investments, closed bank accounts. Once again, prove that your life in the UK is winding down. After all, if you are going to live in a foreign country, you wouldn't subscribe to another year of Sky Sports, you have to consider how the CO will look at your application in this context.
8. The 100% foolproof way of proving domicile in the USA is for your US spouse to leave the UK and go to the US ahead of you. Being apart will suck but it will get you the end result.
9. All of the above have worked for different people at different times. But remember that the person assessing your application may have got dumped last night, might have a bad hangover, or might just be a evil spirited pig.
What you have to do is give them so much evidence they cannot turn you down. They have to satisfy themselves and their employer - i.e. the United States Government - that you will not be a public charge and you are going to have a meaningful life there.
10. Ignore all the friends' advices that by the way, they have green cards and pop over to the States once in a while to top it up. You are exporting your lives and living on a different continent, and you need to be serious about that. Dropping in to the States for a summer of sun isn't the same as living there.
Do remember the most important rule of all is to understand the instructions given to you and give them what they ask for. Each case is different.
Finally, congratulations, you have made it this far. You have got through the preliminaries and some way to achieving what many millions of people will never get the chance to do. Now, you have to close the deal. Get the proof you need, and you will find yourself with a visa stamp and the chance to live in the world's greatest country. Good Luck!
[Original work by Ginblossom]