Hello everyone, I've had issues with lag quite abit in Minecraft, and I thought I could make a post to help people who have the same issues I do, also, this is my first post so don't expect perfection.
 
Green = Highly Effective
Orange = Mildly Effective
Doing all of these steps is (almost) garunteed to increase your FPS!

Install Optifine!
•Get Optifine. Just do it, It's so easy to install, and will always increase your FPS. 
What it's doing: It's giving you more options to configure in your "video settings" tab, allowing you to change them more to your liking and lower, for example you can now choose a distance between "Short" and "Normal" render distance.

Give Minecraft More RAM!
•Assuming you have at least 4GB's of RAM, do the following - Start the Minecraft launcher, Click "Edit Profile" in the bottom left side of the launcher, towards the bottom, you'll see "JVM arguements", check that, and it'll have this written "-Xmx1G"
If you have 4GB's of RAM, like I do, change it to "-Xmx2G" (use about half of your RAM) then select "Save Profile" and run, your FPS should go up ALOT, (mine almost doubled) Note: If you give 3GB's but only have 4 it'll drop your FPS significantly, only give half.
What it's doing: You're allowing Minecraft access to more RAM, therefore more space for it, making it easier to run everything, and increasing FPS.

Turn them Mipmaps down!
•In your Minecraft settings turn "Mipmaps" to 0 or 1.  
What it's doing: Mipmapping makes long distance textures slightly blurred (hardly noticeable)

Update your drivers!
•Update your drivers. Google your driver type, and how to update it. Pretty simple, but will help ALOT.
What it's doing: Updating your hardware to it's latest and greatest software counterparts!

Decrease your PC resolution!
Decreasing the resolution of your PC will heavily up your framerate. To do so on Windows, simply right click the desktop and select "Screen resolution". Even changing it down slightly will help. 

Use a low resolution resrouce pack!
To keep your framerate up, it's reccomended that you use the default resource pack, but any resource pack of 32x32 or 64x64 resolution will help keep your frames up.

Clear up your PC!
•Download "CCleaner", it'll clean up your PC for you and will just help it's overall performance. It's free, as well.
What it's doing: Cleaning up any temporary files and whatnot.

Clear out your %temp%!
•Clear out your %temp% files! To do this, navigate to "Run" and enter "%temp%" you will generally see ALOT of files, these are temporary files and are not of any importance. Just click "Select all" then either drag them to your Recycling Bin or right-click and select "Delete".
What it's doing: Clearing up some unwanted and un-needed files.


Clear up your RAM!
•Clear up your RAM! If you're computer has been on for awhile, it may be slowing down due to idle processes in the background. Follow these steps to easily clear your RAM! This will NOT delete anything you wouldn't want deleted, this just clears up your RAM, RAM is Random Access Memory and is generally cleared with every reboot.
How to clear RAM: First right-click anywhere on the desktop and select New < Shortcut 
Now enter the following line (marked in red) when asked for location of the shortcut and hit "Next":

%windir%\system32\rundll32.exe advapi32.dll,ProcessIdleTasks

Done! Now to use it, make sure no games are running, and double click it. Nothing will open/load no screen or anything, it'll just clear your RAM automatically, takes about one second.
What it's doing: It's clearing up your RAM space, therefore allowing Minecraft take more space until the next reboot.

I'll be adding more later :) Hope I helped!