You purchased some dumbbells and have been working out consistently, you're feeling strong and want to push yourself a little harder, but spending more money is not an option. Can you keep challenging yourself? Absolutely! There are lots of practical ways to progress (to make more difficult) or change your favorite exercises so that you continue to get stronger.
Turn up the Volume. Increasing the number of repetitions, number of sets or both increases the total volume of work your muscles have to do.
Add a Balance. Standing on one leg to do your upper body work or exchanging your regular squat for a single leg squat requires a mind-body connection that adds difficulty.
Make it Asymmetrical. Perform the exercise with one weight, one side at a time and ignite your core muscles. An example of this is a chest press. Instead of doing both arms at the same time, anchor your left arm to the floor and perform the move on the right side only.
Slow Down and Hold. Sounds backwards right? Sure, when you are first learning to workout with weights a slow pace is essential for establishing proper form. But, returning to a super slow pace and/or holding at the peak of your muscle contraction are both ways to increase your muscle's time under tension.
Skip the Rest Area. Shortening the rest time between your sets is another effective way to continue to get stronger without increasing your weight. Consider building a Tabata style workout with your favorite exercises or adding an AMRAP (As Many Reps As Possible) round.