How Can I Extend My Stay in Thailand on a Multiple Entry Tourist Visa?

There is no rule you need to leave a certain amount of time.

How immigration officer considers your history upon your next entry to Thailand is a totally different matter and depends on him/her and his/her mood at that time.

I'm not sure what "I can come again" means. You'll need a new visa, or another way to enter. I'm assuming you're German:

1. Visa Exempt. This gives you 30 days without a visa, and is extendable by another 30 days at the local Immigration office for 1,900 THB. You can get unlimited of them by air (or until passport control says no), and two by land.

2. Single Entry tourist visas. These are good for 60 days, and can be extended by another 30 days at immigration. You may be able to get another one in Laos, but they are getting strict about back-to-back visas.

Of course, you can always go in the more longer term route:

a. Get a job. As long as you're working on the correct visa and extension, you can stay here.

b. Get a bachelors/ masters/ phd degree.

c. Get married to a Thai (bad idea).

d. Have a half Thai child (a really bad idea).

e. Invest 10M THB in the Thai economy

You *MAY NOT* be able to get an elite visa, letting you stay in Thailand for 5 years, because you have the one day overstay stamp in your passport.

Naturally, you do not need an agent to get these visas.

Ok, leave on March 12, and bounce out. You'd get another 60 days which can be extended by another 30.