So maybe you have a lot of mods and your minecraft keeps crashing because you don't have enough memory to run minecraft so it keeps crashing. This is a tutorial for how to do this on a mac and Im pretty sure its the same for PC.
Here is the first step
1) right click on minecraft and scroll down to "Package Contents" and click on it.
2) then once its open go to Contents and choose the file called info.plist
3) open it with textedit and scroll down to the bottom and you will see some text like this <key>VMOptions</key> <string>-Xmx1024M</string.
4) once you find that just change the number "1024" to any amount of megabytes of memory {1024 is a gigabyte}
Dont go to high because then there would not be enough ram to run your computer and any other application open at that time!
This will help your minecrafting needs and let you get more mods with less lag, run better texture packs and just have a better game in general!