Put copies of files, photos, etc from any shared computer onto a USB drive or in a 'cloud' storage.
Take copies of marriage certificate, children's birth certificates.
Gather together your passport, birth certificate, any other important docs.
Scan copies of house deeds, mortgage docs, utility bills, insurance docs, car documents, etc - store on USB stick/cloud.
Separate your finances as much as you can now: e.g. get your own bank account.
Gather financial information: savings, pensions, ISAs, debts, credit cards, insurance plans.
Go through the house systematically. Have a clear out of your own personal stuff: work out what you definitely want to keep and take with you. Start making it easy to move things, so store things in boxes/storage bags, etc. Make it so that when you do decide to go, it's easy to grab your stuff and go quickly. Tell him you're having a clear out if he asks in the meantime.
Go see a couple of solicitors now to find out your rights with regards to the house and finances. Get yourself armed with as much legal information as you can. You can get a free half hour with many solicitors if you shop around. Have a solicitor in mind for when you leave.