Company Relocation Packages US
Holiday Allowance - Don't skimp, you're the one getting transferred for the employer. You get screwed here, you'll get screwed else where...and you'll still get screwed. Other wise you'll be lucky to get a weeks holiday and sick time on top. And why move if you don't get the chance to explore the country?
Temp stuff - Housing and car. Till you get your own place (preferably a rental for the first year) and car, it'll take a while and not be cheap and not having to worry about a hotel will help with the stress of a move. A large company might even provide a relocation specialist and this will be great, especially for those with families as you'll get ideas for school districts, kind of commutes etc.
Tax - An accountant for first year a must, last year a bonus. Also have them "gross" up your relocation package, so that come tax time you aren't left with a scary tax bill on your relocation package which could leave you well out of pocket unexpectedly
Medical insurance - It's a must, but you can try and get them to pay it all or at least include vision and dental
Flights home to the UK - This is a nice bonus once a year, but at the very least emergency flights, your mum or best mate get hit by a bus, getting a flight that night will cost you way in the thousands
Start up cash - This is a nice bonus, but worth asking for, even if they pay to ship all your crap over, you're still going to be buying a new tele and pretty much every electrical item and the very least a few house hold essentials till your stuff arrives, so kitchen utensils, a couple chairs etc. This adds up way quickly
Repatriation - They make you redundant, make sure they pay to get you and all your crap sent back to the UK, it'll make them think twice too which helps with the lack of job security. See if they'll also cover costs if you don't end up liking it here, this is a big bonus as often there'll be a sliding scale where you have to repay the costs if you leave/quite very soon, going out for anything from 1-2 years
Greencard - In writing, they'll get on to it straight away, even if you don't care, you might well do after a few years and by then it might be to late, plus they've got you by the balls until you get the greencard
Other expenses you incur - Have to sell a house/car at a loss, or face losing the deposit on your rented accommodation? See if they will cover the loss. Ask if they can introduce you to a local credit union which might help with starting out with no credit history for insurance/loans