I only host a couple of friends that usually don't have very important data on my server. But i also shelled out for a backup system that comes with the hosting (acronis backup) that backs the whole server up regularly and automatically keeps backups e.g. a weekly backup from last month and a monthly backup from the last 6 months etc.
Also I do have insurance against IT mistakes (though that is mostly for my work with clients on their servers because that could get expensive very quickly if I make a mistake).
The rest comes down to the contracts you have with your clients I'd say. Give them the responsibility and liability you're willing to take on, otherwise you can just help them find a provider that's fitting for their needs and offer a consulting fee for maintenance etc.