Easy tagging, reblog/queue-ing, and more
Lately, I've seen some posts complaining about easily fixable issues on Tumblr, so I thought I'd pass on knowledge that used to be common but seems to have been lost along the way.
Xkit is your friend. Specifically, XKit Rewritten, the browser extension.
Download it to your browser, and you can get rid of ads, turn on the mutual checker, timestamps, quick reblog, and quick tags ... and that's just for a start. If you're not familiar with extensions, just go "Add-ons and themes" in the Firefox menu and search in the top bar for "Xkit Rewritten." (And yes, it's in Chrome, too.)
Quick Reblogs & Quick Tags - a match made in internet heaven
Quick Reblogs and Quick Tags work in tandem to give you the easiest set up for reblogging/queueing posts with appropriate tags. It requires setting up "tag bundles" at the beginning, but it's a simple thing. Just give the bundle a title and then list whatever tags you want under the title, separated by commas. Click "Add Bundle" when you're done.
You can make as many bundles as you want.
This pairs nicely with Quick Reblog in that you can add these bundles from the box that appears when you hover over the reblog button on ANY post. Just click the tag bundle(s) you want to apply, add any additional/specific tags if you want (or you can also just write your own tags into the "Tags" section without using a bundle at all) and then click "reblog" or "queue" for a one click solution without ever leaving your dashboard.
You can combine bundles, too. So if you're an artist/writer, you could make a generic bundle for your general art or writing tags, then make specific ship/fandom bundles and combine them based on what you're posting. I tend to pick a ship and stick with it for years, so I don't really need that, but for all you extra-awesome multi-fandom people, this can be a huge time saver.
There are a ton of other options with XKit, too, but these are the ones I find most useful. I hope this helps people feel more confident using the site and also tagging posts!