Putting Sponge and Topsheet Together
As I mentioned earlier, most table tennis rubbers come premade, with the sponge and topsheet already joined together. There is an almost bewildering range of rubbers to choose from these days, with all of them manipulating the factors I have mentioned above to produce their own unique effect. There are also a couple of points to think about when the sponge and rubber are combined.
Throw Angle
One more variable to consider is what is called the throw angle of the rubber. This is whether the rubber tends to 'throw' the ball higher or lower when making the same stroke. Given the same return from your opponent, and the same stroke by you, a rubber that puts the ball in the net is considered to have a 'lower throw' than a rubber than puts the ball on the table. For the same circumstances, a rubber that puts the ball off the end of the table is considered to have a 'high throw'. You will sometimes hear players talk about 'degrees of throw' - basically a rubber than puts the ball low has a low degree of throw, and vice versa.
This throw angle is not an exact science (contrary to what you might think from reading on the various forums!), but more of a comparative feel between different rubbers. A rubber with a high throw is not better or worse than a rubber with a low throw, it is just different. Generally, low throw rubbers are considered to be easier to play with against topspin, while high throw rubbers are supposed to be able to lift backspin over the net more easily.
Weight
As mentioned in my article on choosing a blade, you may also want to be aware of the differences in weights between rubbers. Rubbers with thin or no sponge will tend to be much lighter than those with thicker sponges. Speed gluing also adds to the overall weight of the bat, since you are adding glue each time you play. Some of the solvents in the glue will eventually evaporate, but there is always a residue that will continue to build up over time.
Choosing the Rubber
The best advice I can give you when selecting a rubber is to talk to an expert first - such as a coach or advanced player that you trust. Get them to watch you play and ask them what they would recommend for you, and why. Don't be afraid to get a few opinions if you can.
Once you have some ideas to work with, see if you can try out the rubbers. Perhaps you can find another player who uses these rubbers, and borrow his bat for a hit. Better yet would be to borrow the rubbers from him, and then try them on your own blade - it's amazing how different blades can affect the feel of a rubber. You might consider trying to start an equipment pool with fellow players so that you all can share rubbers for testing purposes.
When testing the rubbers, try out all your shots, and then play a few matches with them. See whether you like the feel of them, and think about how they compare to your old rubbers. Remember, you are looking for a rubber that will suit the style of game that you want to play - attackers won't need a rubber that can chop well, and most defenders won't need a rubber for fast attacking!
Finally, once you have chosen your new rubbers, give yourself some time to get used to them - it will take a while to adjust to the characteristics of your new rubber. Don't give up too soon on your choice - give it a few weeks at least. Hopefully though, you will love your new table tennis rubbers and soon be playing even better with them!