This strat is easy as pie and will definitely get you to the top.
If we're being honest, Slither.io is way more fun when you're an aggressive snake. Being aggressive is generally the way to go when you're small, but once you're touching 5000 length and above you should shift to playing a little more conservatively to protect yourself. This is especially the case when the servers are lagging badly.
There is one way you can secure your size and grow longer in Slither.io with minimal risk. It will get you to sizes you've never reached before! But it's perhaps too easy.
This is the one "hard" step and it's not even that difficult if you take the advice in our two guides on being the best snake you can be in Slither.io and being one with the lag.
You don't necessarily have to get to at least 10,000 size, but it is recommended to make the most of this strategy.
Get your kills and munchies anywhere, but once you hit 10k it's time to migrate to the center of the map.
A lot of players naturally wander to the center of the map thinking it's the most populated, and it generally is since people assume it to be. You need to be somewhere there's a lot of foot (snake?) traffic, and the center is it.
Since you should be at over 10,000 in length, you're long to be a pretty long snakes. And long snakes can make big circles.
You want to loosely coil yourself into a big circle. Sort of like coiling around other snakes for a kill, but without the excitement of fresh meat for the taking.
You should be keeping your head on or very close to your tail, only moving a bit inward or outward for loose particles floating around.
With this method you keep yourself more safe than you would be just slithering around. That's a big bonus here -- but the real bonus lies in just how often other snakes will kill each other right next to your body, run into your body, and eventually spawn within your circle.
After you get above 30,000 size (which will happen if you do this for long enough and don't get greedy) other snakes will start spawning in your circle fairly frequently. These are free food as they have no choice but to either log out or eventually slither into your body. After all, they have nowhere else to go.
Just like when you're coiling around other snakes specifically for a kill, other smaller snakes are going to try to target your face. This is why it's so important to not let your face get too far away from your tail.
The amount of tail overlap you should have varies from situation to situation, especially when lagging. If you're feeling safer, less tail may be better. If there are a lot of aggressive snakes around, more may be ideal. It's up to your playstyle, but don't stress over it too much. Just don't let there be a gap for smaller snakes to try to ambush you through.
Don't get greedy! Yes, being aggressive and going after chunks of stray particles is super fun. But it's also risky and is the only way most people die when using this strategy. Be aware of your surroundings and be sure it's really what you want to do before breaking your circle formation.
Don't get upset if other snakes grab particles from snakes that have run into your body. It happens, get over it. Don't take too many risks if you want this strategy to work. You'll get plenty of other opportunities to eat, trust me.
This strategy may take time to get to pull off and to get used to, but it's great if your only goal is to hit the top of the charts. You'll be surprised at how long you get, but you may not be all that entertained by getting there.