The Random Access Memory you have in your mobile is Fixed.
So, you can't increase it but by proper utilizing it you can use your phone more effectively.
1) If you have a RAM of 742 mb or more than that than try 'Clean Master'.
it will clear down your RAM and also delete the junk file which slow down your phone. This app can also run in phones which have 512 RAM but remember it will take 30 to 40 mb of RAM.
2)If you have a RAM of 512 mb or less than that than try 'ES Task Manager'.
it will clear down your RAM and junk files. It also provide some good additional features like power optimizer,device information,system info etc.
3)but if want to clear your phone manually than try following steps-----
Go to settings --> Application Manager --> Running --> select the app you want to stop --> Stop --> OK.