General Tips and Tricks
Agar.io Guide
Your cell's size has direct effect to its speed. As it grows bigger, it will become slower. While you won't be able to chase smaller cells, your size alone will make anyone smaller than you think twice of approaching you.
There are neutral green masses present in the screen where you can go and hide. This is a good way for smaller cells to seek refuge when chased by hungry and bigger cells. In the other hand, if you're a large cell and you happen to collide with that mass, your mass will be scattered to several smaller pieces, making you vulnerable from any nearby larger cells.
Splitting lets you sacrifice a portion of your cell, making the attacker a bit larger (and slower) while making you smaller and faster. Avoid splitting when surrounded by several smaller cells though; dividing yourself in smaller chunks makes it easier for them to absorb your discarded portions.
Once your size grows substantially, split your cell to at least 4 parts so easily gather the particles scattered in the screen faster by covering more ground.
Always stay away from the edge of the giant cells. Also, don't hesitate to run if you see a larger cell moving toward you. Be careful of docile-looking cells that may not seem to show any hostility since they can still ambush you by shooting a part of them to absorb you.