I use WordPress and WordPress websites need regular updates, which can bring about conflicts between plugins or even some plugins ceasing to work. In such situations, it can take many hours to fix the problem. How to charge for these hours? Do you specify it in the contract, and if so how should I present it?
I tend to avoid fees as much as possible, and I also tend to avoid providing hosting services (or similar) for my clients. I'll certainly manage their hosting / backup / whatever they want at my hourly rate, but I don't want to become a reseller of someone else's products.
(As an aside: I realize it can seem lucrative to markup and resell services like hosting, but it's easy to forget there is still a cost - now your client expects you to fix whatever is wrong with the service, not the original provider. You're no longer a freelancer, you're now a web host / email host / whatever - you just have very little control over the service you're selling.)
If it's something that requires some kind of active observation on your part (in this case, keeping an eye on WordPress updates), I've found a kind of retainer with a few included hours is a reasonable way to package it. That way your client knows what the cost will be each month (I'd just be sure to present this as separate from any hosting, license, or similar fees they pay to a vendor) to have you manage their installation of WordPress.
When there's some significant change that requires more than the include hours, you can move to hourly billing, or just take a look at the last 6 months or so of work. If you're including 3 hours in the retainer fee, and you're only spending 30 minutes a month - it's probably best to keep the clock off (at least for a while).
You should negotiate to client and set up X work hours per year which he pays you in advance (for example, 20 hours in 2013). Until you spend these X work hours, your client will be prioritized and may reach you 24/7 (not literary) to start working on fixes. This is what big companies do and what you should do as well IMHO.
After he spends the annual work hours, then you can also work for him hourly but you are no longer obliged to make him prioritized client.
But make sure you're client understands all this 100% since you don't want him to be surprised with you non-immediate-availability after he spends the pre-set number of work hours.
You may check up Retainer Agreement.
Do you have a maintenance contract in place with them? is it a set price or is it based on an hourly amount?
These are easily covered in a simple contract, you can charge monthly for X amount of hours and then X amount for any time above that or you could charge a set rate and take the hit if it takes longer one month.