How to Get a Ton of Followers on Tumblr
1. If you really want a LOT of followers, DO NOT run a personal blog
As much as we all love our personal blogs, filled with eclectic pictures and random thoughts, these blogs are not the ones that get 10,000+ followers. In order to get a huge amount you must have a niche blog. On Tumblr right now there are a ton of "pastel" blogs, cat blogs, blogs dedicated to bands, blogs full of Simpsons screen caps, etc. These are the blogs that get the heaviest amount of traffic because people are searching for specific keywords and tags.
2. Tag the hell out of your posts.
Whenever you post an original post, tag the HELL out of it! Tagging your posts are the only way to get mass views and mass amounts of followers organically. Say you post a picture of a rose, do not just tag it as "rose", instead tag it as "rose", "flower", "red", "pretty", "plant", "nature", etc. This opens up a wider playing field and allows more people to find your blog. Note, you do not need to tag reblogs because those tags will not direct anyone to your page, the will only organize the posts within your blog by tag. Please also remember that if you post an image that is not yours DO NOT credit it to yourself and always try to credit the true source. You can get into a lot of drama with people over stolen content on Tumblr.
3. Promote your Tumblr with sites like Tumblring.com
Although this isn't a great way to gain followers, it's another avenue to try. These types of sites work better with themed blogs and with "follow for follow" bloggers. Of course, the downfall of sites like these are that most of the people on there are looking for followers, not people TO follow. So a lot of the time people from those sites will follow you expecting a follow back. Which brings me to...
How many followers do you have
4. Follow for follow and promo for promo
These are both moderately effect ways of gaining followers quickly, but there are drawbacks. When you employ the policy of "follow for follow" you end up following 80,000 Tumblr users and your dashboard is clogged and full of posts you would never reblog. Another sticky situation that arises is that many people take advantage of bloggers who follow back automatically, bu getting the person to follow and then unfollowing said person a day later or so, and then you're right back where you began in terms of followers. "Promo for promo" is less of a crap shoot for getting followers, but you end up with many followers simply because they wanted a promotion, not because they liked your blog. Also remember that many people scroll past promos on their dashboard without glancing at them once. There are also promotional contests that more popular blogs have such as, "first ten to reblog get a promo", but I've scarcely seen results from this method.
Seriously. I know everyone says this about gaining followers but it really does help. When you follow someone and they follow you back or vice versa, you have immediately made a bond and a connection that says, "we have similar tastes. I like what you like." When you mutually follow each other, both of you are less likely to unfollow either one, and you are also more likely to reblog each other, thus ensuring yourself a small amount of promotion from their blog. The best way to gain free tumblr followers that stay are followers that follow someone who follows you (convoluted, I know).
6. Have a nice, clean theme.
There is nothing more annoying than scrolling through someone's blog who still has freaking redux as their theme. It's as if they've never explored the site at all, never clicked the customize button, and have never even looked at their own blog-- and who wants to look at a crappy blog! It's frustrating and distracting and anyone with the slightest knowledge of HTML will laugh at the shabby conditions of your blog. I know this sounds harsh, but people on Tumblr are tech savvy, they notice these things. There are also millions of free Tumblr themes easily available to anyone.
7. If you have a personal blog, have an about me and a tagged/me
People really do hate faceless bloggers. Most people look for your "tagged/me" link immediately upon visiting your blog so that they can put a face to the posts. People want to form a context, a framework for what they are viewing. Note, a "tagged/me" is your page of pictures of yourself that are tagged with the pronoun "me" so they are easily accessible.
In general also remember to be mindful of others on Tumblr. In the end it doesn't really pay to be mean or to lie (look up user "vaginastew" for an example of that). A lot of people on there are genuinely nice people who want to follow other nice people.