How & When To Customise A Racket
The only part of a racket that can't really be changed is the actual frame itself (well, in theory). Other than the frame, anything's fair game.
Many players who buy a racket may find that the grip doesn't quite feel like their last racket, that the frame feels too head light/heavy or they are getting too much power and not enough control or vice versa. There are a number of options, including:
You can build up a grip by using a wrap, which is designed to shrink fit around the handle of your racket once heat is applied. You can add layers to make thicker, rather than putting grips on top of grips - which can ruin the feel of a racket as the feeling is over-absorbed through 2 or more grips. There are also simple fixes, such as creating ridges on grips by overlapping - a fairly common theme in squash, where players often need a little more traction on their grip due to very sweaty hands!
Aside from the strings making a big difference to your racket, choice of grip can also be vital as minimising unwanted slip on the grip maximises a players control of the racket. Here are some options:
PU Grip - Typical 1st choice for most racket players, will typically be the only grip on a racket. Cushioned, with a tacky feel. The class leader in the market is Karakal with their PU Super Grip, though there are many similar brands. You can get a variety of PU Grips with additional ridges, more cushioning and breathable wholes. Most pro's just use a simple PU grip.
Over Grip - More typical in Tennis than in other racket sports, over grips are thin, and designed to sit on top of the regular, cushioned, grip. Often very tacky and designed for maximum grip in hot, sweaty conditions. They can be removed quickly and replaced quickly too. You can also get over grips which are dry feeling and non tacky. There are many options, but personal favourites are Yonex Super Grap (tacky) and Tournagrip (dry).
Leather Grips - Only really used in Tennis now as they don't lend themselves too well to rackets with smaller handles (they can be very slippy). Good in Tennis, as handles tend to be wider. They can also last a bit longer. I'm personally not a fan of leather grips, but others are.
Toweling Grips - Old school and every now and then they make a comeback! Works like a normal towel, wicks away dampness and allows fairly good grip. The most effective way to grip a racket until the mid 80's when PU grips became the norm. If you use these grips, ensure you air your racket for a few hours after use before putting back in your bag!
You can buy 3 identical frames from a manufacturer and each racket may have a different weight and balance. Find the racket that you like the best and you can then make adjustments to others. It's very rare for every frame to be balanced exactly the same. Using lead balancing tape at various points along the racket to create a new balance point is a great way to instantly change the feel of a racket. You can also use lead tape under the grip to create a more head light feel or use silicon/resin and other materials inserted into the handle, which can also help to dampen vibration, as can vibration dampeners, which are more readily used in Tennis and Racketball.