One way to significantly impact your bike's performance is to upgrade the wheels and tires. If you can drop the weight of the wheels and tires even slightly, you'll notice a definite difference when you bike.
Many options are available for upgrading your bike's wheels. Major wheel manufactures offer products with a variety of rim styles made of different materials. Wheels are also available with an assortment of spoke designs. Because of the improved quality of spokes, wheels are being built with fewer spokes but with comparable strength and durability.
If you upgrade your tires, you'll realize another performance gain: Higherquality tires provide reduced weight and less rolling resistance, which makes a dramatic difference in performance. Better-quality tires are designed with more threads inside the rubber, which holds the tire together, allowing for greater air pressure, less contact surface, and a fast ride.
The tread pattern on the tires also has an impact on ride comfort. The less tread on your tires, the less rolling resistance and the better performance you'll experience.
Most wheels allow you to change the width of your tires. Tire width impacts performance — narrower tires give you a faster ride because of their decreased rolling resistance, and wider tires are more comfortable because they absorb more road shock (although wider tires have more tread on the road and more rolling resistance). To determine the width of your tire, look at the dimension written on the sidewall of your tires; specifically, look for the second number. A marking of 700x25 means the tire has a circumference of 700cm and a width of 25cm. If you plan on changing the width of your tires, work with your local bike shop to make sure you find tires that work with the dimensions of your frame.
The other important consideration when choosing tires is durability. If you're touring and riding a significant number of miles with fully-packed pannier bags, you'll want a very durable tire that can handle the weight. A more durable tire will also lead to fewer flats along the way. Many of the tires specially designed for durability have a Kevlar belt or other material designed for protection.
If you want protection from flats but don't have the money to upgrade your tires, you can fill your bike's tubes with a puncture sealant. The liquid inside the tube will automatically fill any punctures that may occur. In addition to puncture sealant, you may want to opt for thorn-resistant tubes (which are thicker than normal tubes) and tire liner (a protective stripe of Kevlar that you place between the tire and tube). The downside to this flat protection, however, is significant added rotational weight — but if you absolutely hate flat tires, you won't mind trading some performance for convenience.