what do you do with your maaser money? do you send off a check to tzedaka every time you get a pay check or keep it in a specific amount until you want to donate it?
how often: bimonthly? monthly? yearly? other?

how do you decide where to donate and/or how much to give certain places?

I send out checks once a month.

I used to basically give to anyone but now I only give to organizations whose hashkafas/activities I agree with.

It has changed over the years for various reasons.
In general I make a cheshbon every so often (it used to be bimonthly, but unfortunately it's not as frequent nowadays due to time constraints)

I then take the total and take a list of all local (we keep our $$ in the city) tzedakah organizations and we give out based on what org does? needs? etc.

For example, to the organization that gives poor people bundles of food on a weekly basic and is run by all volunteers we give more than we do to the mikvah org. (This is an example of what I can see myself doing, not that I have any specific recollection of the choices we make)

Lately, however, to keep things simple we'll pick one org for that check and maybe a different for another check or so on. (when i say 'we' I mean 'me, figuring mh just would rather me do it than bother him about it')

that's the basic idea of what I do.

I have a tzedakah account into which the ma'aser goes immediately. That way its less likely to be spent elsewhere and forgotten about.

G-d usually extracts it from me directly...

After each paycheck, 10% is automatically calcuated and put into a file. Each time we write a check for maaser, we subtract from the total amount.

I round up my maaser to a nice round sum and know I owe that amount a month. All my maaser goes a specific poor family. Right now I'm saving up to cover one specific expense so I have a few month lapse in paying it. I usually pay once every couple months.

I just figure that I need to give 10% of my paycheck. I keep track of what I give (need it for tax purposes to) and at the end of the year I look to see whether I gave 10% of what I got paid that year, and if I didn't, then I add. I don't like having a separate account.

It's interesting how widespread maáser kesafim has become. Appropriate for an affluent society.

That's very nice and well-intentioned, however there is an inyan to give exactly 1/10th.

I split the money into 2, 1/2 going to one institution, 1/2 to another.

What makes you say this?

What do you mean?

Was it uncommon in other times?

Yes. It's also questionable if it's a rabbinic mitzvah or a minhag.