I have my Patreon set to charge upfront, so as soon as someone signs up they get charged for it, and then they're charged again on the 1st of the following month, and then that reoccurs monthly.
Before I changed to charge upfront, and had things set only to monthly charging, I would wait until that month's payments had gone through before I send out rewards, just to confirm that the backer actually paid for things, instead of doing that thing where they sign up, get the thing and then cancel their pledge before they're charged. Happily, that's no longer a possibility!
For downloadable rewards, I tend to message them with the link to the folder where I keep those things within a few days of them signing up to be my backer. And then I do a mass-PM to the backers on the relevant tiers when I add anything new to those folders. The downloadable rewards are a handful of desktop wallpapers and a bunch of PDFs of comics; some backers have access to all of it, other backers only to certain things, so I've got them divided up in different folders.
As for content posted ON Patreon, I tend to do early comic pages on the same days I update my comic for regular readers (mon/fri, with patrons being two pages ahead), and then I do a weekly sketchpost/misc. on Wednesdays.
ETA: I also do occasional Q&A's and between-chapter sketchcomics that are Patreon-exclusive; those tend to go up on Wednesdays too.
I have backers spread out across all tiers, so this is not an issue I have to deal with at the moment.
Twitter, Tumblr and Tapastic. On Twitter and Tumblr, I tend to mention it a.) when I alert people of updates (on Tumblr, that's 1-2 times per update day, on Twitter, 3-4 times), b.) when there's a specific event in which it is relevant (#ComicBookHour, for example), and c.) maybe mention it a few times a month randomly, and usually at the beginning of each new month. Just remind people that it's there.
For context: this is what my announcement-tweets look like. An announcement of the new page, with link to main site/Tapastic, and a mention that I've got a Patreon, as well as a crop of the new page in question. I then bump or RT that tweet a couple of times throughout a 24 hour period after the update's gone up.
I also have it linked in my various profiles, and on Tapastic, my comic's headers and side-bar banners link to the Patreon-page as well - and I occasionally mention it in the author's comments.
Here's what my Patreon page looks like at the moment! I'm actually doing my first Patreon-exclusive livestream tomorrow, which is fun.