It's all about appreciation

While it's unlikely anyone will START donating to you because of your thank yous, it's extremely likely that they'll KEEP supporting you if you show appreciation. It also encourages them to support others, so ultimately, the more they feel good about helping you, the more they'll keep doing it.

The main ways of showing appreciation:

The tenets of a good thank you

Sample Thank Yous
Make it personal

Peter Hollins refers to his fans as the "Hollins Family" which is good because it's personal, but bad because it's vague.
So if Peter Hollins were to do a thank you video, the message would be "Thank you for being a part of the Hollins Family".
Because now, they know they're a part of the group.
It should almost feel like they're being pulled aside for some special appreciation

Use the format they know you as.

We want the thank you to come from the source they know you as (otherwise it might seem odd). So videos for celebrities and youtubers, comics for comic book artists, etc, etc

Genuineness over fanciness

The tempation might be to do something big or fancy to say thank you, but its not as meaningful as something that is real and heartfelt. You should drop the act and show genuine gratefulness while doing the message (unless you're literally an act, in which case, maybe your character should drop the act).
Keep it short and sweet.

Repetition is ok, copying is not.

The nature of monthly thank yous is that you're probably going to be saying the same thing over and over, and it's important to know that that's perfectly ok. They're not expecting creativity, they're expecting genuineness.
What's NOT ok, is to reuse your videos or images. The idea is that you're taking time out of your day to say thank you, and if you just copy and paste an old message, it just says you don't care about them.

Keep SupportStream out of it

SupportStream was built for YOU, to help you succeed. And while I certainly appreciate any thank yous, your message to the supporter is not the place.
It's important to keep in mind that we're not the ones giving you money, we're just the system that they use to do it, and we don't want to do anything that deludes the value of the appreciation they're getting.
So, if it comes off as "sponsored by SupportStream" then its going to look like we made you say thank you (which takes away from it).
And if you thank us as well as the supporter then they have to share the apprecation with us, which takes away from it.
So, by all means say thank you on twitter or separately, but when it comes to the supporters, it's just about you and them.

If you're a charity or a cause

If you're a charity or some kind of service that people appreciate then it's important to remember that it isn't really about you, it's about the cause (although personalization is always better). So here are some things to focus on in regards to the cause.

The messages should be from the person who has accountability for the progress, (so a CEO and not a spokesman), and should be in much the same format as a shareholders meeting.
Show progress

It doesn't have to be grandiose, you don't have to cure cancer every month, but you do want to show that you're making strides towards that goal. It can be hiring people, small breakthroughs, anything that says "we're putting your money to good use"

Honesty is ok, but keep it positive

The nature of what you do is that there are probably going to be setbacks. It's ok (good even) to be honest about them as it lets the supporters get more invested in the cause. But it's also imporant to keep the message positive. So there are problems but you WILL overcome them.

Remind them they're a part of something

It may not be much of an investment on their part, but it does mean something to know they're a part of something bigger and they're helping that cause.

You can ask for help, but NOT money.

If you're struggling with something, one of your supporters may be able to help you out, so it's always worthwhile to ask for such things. BUT this is not the place to beg for more money, so please keep it out of your messages.