I was recently thinking about the same thing. I recommend starting small and giving to every cause you like, including causes that don't have 501c3s. Start with a small amount, maybe $1k or so, and the same amount for everyone. They'll all want to treat it as annual, so don't overdo. Then once you know them better, decide whether you're happy with it, want to increase it, or want to cut it before they ask for an annual contribution.
I also pick one month of the year for all of it so you don't make 3 "annual" contributions a year. My parents used May, for tax simplicity I think, whereas I use December so it can crowd out with giving to people I know.
This is assuming you're looking to give primarily. If its largely a tax question ask an accountant. Just remember, "they can only bury you with one set of golf clubs." Once you have more than you or your family will ever spend on yourselves, better for you to dispose of it than your heirs.
I'd like to hear from people about a process they use to decide how much to give to charity. Personally, I have very high income, and while we donate significant money in absolute terms, in relative terms the amount is <1% of our post-tax income. It seems to me that it's too little, but I have no moral intuition as to what the right amount is.
I have a good intuition on how to allocate the money, so that's not a problem.
Background: I have a wife and two kids, one with significant health issues (i.e. medical bills - possibly for life), most money we spend goes to private school tuition x 2, the above mentioned medical bills, mortgage, and miscellaneous life expenses. And we max out retirement savings.
If you have some sort of quantitative system where you figure out how much to spend on charity, please share. If you just use vague feelings, and you think there can be no reasonable quantitative system, please tell me that as well.
Update: as suggested in the comments, I'll make it more explicit: please also share how you determine how much to give.