Depending on the age of your rats will depend on how easy introductions happen. Older rats tend to be more territorial and will fight more readily for this. Introducing younger rats to younger rats will tend to be more easily accomplished, depending on how long each has come to get to know its own territory. Introducing a young rat to older rat( can be difficult not just for the territory aspect, but also because of the sheer size. Older rats seem to see the young ones as easy prey and will pick fights with them more easily. The easiest introductions would be two young rats and the one you will probably find most difficult would be two older males, especially if they have not been neutered. But introducing rats of any age can done, as long as you have the time and patience to see it through.

1. The first thing you need to consider when trying to introduce two rats to each other is a separate cage. NEVER just put a new rat into your own rats cage straight away as your resident rat will obviously fight (sometimes quite harshly) over their territory. Put the new rat in the separate cage, but set is as close to your own rats' cage as possible (not too close that toes and noses can be reached as they may get bitten). This will allow the new and resident rats to get to know each other smells a bit before introducing them.
2. You may wish to try putting the rats in each others cages, so that they can smell thoroughly and will leave their scent for the other to get used to. Try this over a period of time, especially if there are aggressive tendencies of any of the rats (puffin up of fur, hissing and spitting, backs arched, etc).
3. If their doesn't seem to be any aggressive behaviour on the parts of either sets of rats then you may start trying to introduce them to each other in neutral territory. A good place may be the bath, as it is probably the most unlikely place your resident rats will have left any smells. Please note it is always a good idea to have a thick towel handy to break up fights (where actual damage is being done to one of the rats, rather than just dominance fighting where they will try to pin each other down). Another good thing to have handy is a small water sprayer. Rats (at least my lot!) hate water and it will scare them for that short time to stop fighting. Squirt them enough and they are sure to get the message at some stage (although some rats will get used to this and not let it interfere!). You may wish to do this stage of the introductions for short, frequent periods if there looks as though one rat may be extremely rough with the newcomer.
4. Again, if step three seems to have been a success, it is time to move the introductions to somewhere where your resident rats are used to playing. This is normally where the violent fights will break out, as the residents are now on their own territory and the newcomer isn't used to being there. Again make sure and have a towel handy and maybe even the water sprayer. A good place to try this would be to set your rats on your lap. By doing this you are showing your resident rats that you have accepted the new rat quite openly, and it may help them to accept it also. Another good tip is to try the dominance technique that your rats use. If a rat seems to be overly aggressive towards the new rat, turn them on their back and hold them there for a short period (this is their way of showing dominance and may work to show that you are actually more dominant than them).
5. Once all the above seems to be working perfectly and the aggressiveness is just dominance rather than actual fighting, you can now start to try and introduce your new rat into the cage. First, clean out the resident cage thoroughly in an effort to remove all the other rats' smells and make it more neutral territory. Even rearranging everything in the cages can help, as this will confuse your own rats a bit. Now try introducing the new rat into the cage. The best time would probably be when they all just want to sleep rather than when they are more active. Hopefully the new rat will be accepted. If not, you may want to try disguising everyone's smell but putting some vanilla extract on their backs. Some people have agreed that this works and if not a more drastic measure (and thankfully one I have never had to try!) is to put a small spot of your own urine on their backs to disguise their smells.
6. Now that you have the new rat in the cage, watch them carefully. If there seem to be signs of fighting, don't rush madly over and lift the new rat out. Try to determine what kind of fighting it is. If the new rat is just been tumbled over onto its back and pushed around, then this is dominance fighting (usually done by only the more dominant rat in the cage, although others may give it a go, just to try and establish the pecking order). If the fighting looks more aggressive and there is biting, then you may lift the rat out.
7. If the fighting stops or stays at just being dominance then you can congratulate yourself on just having made a friendship. If the fighting never seems to die down at all, it is more than likely that the rats will never get on, and should be permanently separated from each other. This usually only happens in very small cases, but it is something to consider when you buy a rat.

Quick Tips Guide

1. Have a good thick towel handy to prevent yourself from being hurt when breaking up fights.
2. Buy a small water sprayer to quirt the rats when they are doing something they shouldn't be.
3. Always introduce on neutral territory first.
4. Show the rats who is boss. When one seems to be acting very angrily toward the newcomer, turn that one on its back and hold it there for a few seconds. This will show them that your are actually the dominant one and may help them calm down a bit.
5. Place a drop of vanilla extract on the rats' necks (at the back of the head) to disguise their own smells. They should then be too interested in sniffing each other to fight!

Once thing of note that I have found handy in trying to introduce females is when they go into heat. My then eldest, Rollie (even though she was over two years old) still went into heat. She was also very aggressive towards any newcomers we tried to introduce, usually going for "the kill". With our last two introductions (at the same time) we were starting to lose hope of every getting them in the same cage. So, one night, out of sheer luck, Rollie went into heat, quite strongly - the slightest touch on her back and she had to stop and vibrate! So, hoping like mad, I put the new rats on the bed with her. And it worked! Every time she went over to them, they touched her when sniffing and she stopped to vibrate. She was so busy doing this that she never had a chance to turn and bite, so she got used to their smell and after about half an hour they were all running around happily together on the bed.

Of course, this technique won't work if you have male rats, but if you have a female, who is really aggressive, why not give it a go?