You need to employ tactics even when you are not in a clan game playing with your team mates.
You have to know the maps you play on, know where the weapons are and where you can get powered up as fast as possible.
Make sure you find a route that you can follow through the levels to get the best Guns and Armor.
When in close combat make sure you are not using the Rail Gun unless you are an EXPERT Railer. Change to Rockets, Chain Gun, or Super Shot Gun. These Guns are very good close up , try and have them close to hand in your customised keys (see Config Examples).
When moving around in combat use the strafe keys, use the forward then left/right combonation (an L shape). Say a jumping opponent jumps infront of you and fires a Rocket where you are standing, strafe to one side and move forward and shoot him while going forward.
Dont make patterns when you are moving, try and keep your movements random, cause a good player will notice these movements and you will find yourself dieing a lot more. A balance between your movements has to made, against a crap player go forwards a to the side while shooting, against a LPB who is shit hot, stay away from them and take pot shots from a distance. The figure of 8 is a good pattern to move in as long you make sure you varie the diameters of the circles you make.