To echo the others...

Don't leave home without it. It's just not worth the risk.

A few things to mention:

You need to arrange it before you leave your home country. Most policies do not allow you to extend it from abroad (as you are usually taking out a new policy once the term has expired and the rules state you have to be in your home country when you take it out.) There are a couple of providers that have got round this and you can effectively extend on the road but you have to take it out initially when you are in your home country.

Most annual policies only cover individual trips of up to 30 days (sometimes up to 60) after which you must return to your country of residence. As Moggy said you have to be able to prove that the planned trip was for less than that period to be covered. The credit card insurance mentioned above will not be valid in a long term travel scenario. You wouldn't even be covered for an incident on day 1 of your travels if you can't show your planned trip is within the allowed period.

Try searching for gap year cover. I got good cover for two years with Amex, at a reasonable price but it couldn't be renewed from abroad.

Make sure it covers medical evacuation, that's what you're really paying for. Don't get bogged down in cover for belongings etc.

As Moggy also said declare any relevant info. They will often still cover you if you have a pre-existing medical issue.

It gets increasingly hard to get long term travel insurance the older you get. A lot of policies only cover under 30s and a few go up to 50. After that it's really difficult.

Good luck.

PS Rules and regulations regarding insurance differ depending on your country of origin. The above applies to the UK (and most likely other countries in the EU).