Since a lot of people are suffering from arm problems like sore elbows, shoulders and wrists they start looking for softer, more flexible racquets (more info about stiff vs flexible racquets). What happens when you go softer is that you lose power. But there are ways to tackle this of course.
1: String choice.
Strings have different power levels. Most professional tennis players use either a quite dead polyester string (Luxilon Alu Power, Solinco Hyper-G, Babolat RPM Blast, Yonex Poly Tour Pro are all popular options on the ATP tour) so they can swing away and depend on the ball landing in. If you have a really low-powered racquet you might want to look at a more powerful string. Something like a softer polyester such as Volkl Cyclone Tour, Solinco Tour Bite Soft or Weisscannon Silverstring or a multifilament string (usually gives less spin) such as Tecnifibre X-One Biphase, Wilson NXT or Prince Premier Touch or Gilles Simon’s choice Head Intellitour.
2: String tension
If you want more power, you should string low. How low you go depends on what racquet you use and how much power you want out of it. Stringing low comes with a loss in control, so it’s important find a good balance. This is an experimentation phase of course. But more and more players are stringing low, so don’t be afraid to try something like 20 kgs or lower (Jack Sock strings his racquets at 40 lbs, which is close to 18 kg). My own personal sweet spot is around 22 kg, while Dustin Brown strings his Babolat Pure Drive at 35 kg!
3: Lead tape
If you’re not happy with how your racquet plays, remember that lead tape can change the characteristics quite a bit before you go out hunting for a new one. Lead tape at around 12 o’clock moves the sweet spot upwards and adds power. Lead at 3 and 9 gives you more stability and power, etc. If you want your racquet’s balance to stay the same, remember to also add weight in the handle since lead on the racquet face makes the racquet more head heavy and increases the swing weight. You can add lead on the handle or add silicone or blu-tack in the butt cap (not all racquets have an easily removable butt cap however) or add a heavier base grip such a Volkl leather grip which weighs 30 grams. I like using Tourna lead tape, but there are several choices out there.
Good luck on getting your racquet to play the way you like it!