Group riding is more fun. Getting together with friends is great for breaking up the monotony of training rides. It is easier to commit to riding when you are going with other people, and can also spark a little competition. Local bicycle shops are a good place to look for group rides. If they don't host their own, they can probably point you in the right direction. Remember - at the end of the day, cycling is supposed to be fun!
Be prepared. It's never good when you get a flat and don't have the equipment or the know-how to fix it. However, this tip isn't only about mechanical issues. It's also important to have plenty of food and water to last the entire ride, and to have the right equipment. Do some research to be sure that you have everything you need before your next riding adventure.
Safety is key. Always be aware of what's going on around you. Remember that if you go head on with a car you'll probably lose. It's never a bad idea to have some lights on your bike to be a bit more visible especially if riding alone. When riding in groups, be sure to point out potholes, sand, and water on the road. In the trails, point out low branches, debris, or washed out areas.
Racing is great training. Be on the lookout for races in your area. You can meet some awesome people at bike races, as well as push yourself to be even better than you already are. Riding new trails is a great way to work on your reflexes and handling. Also, riding new roads can help you experience terrain that you haven't before like steep climbs and technical descents.
If any of this seems foreign to you, remember that the internet is an amazing place. Also, cyclists are generally friendly people that are willing to talk about almost anything bicycle related. Now get out there and ride!