Cyr -
An old trick that works even with new HTs is simply making a counterpoise for the antenna.
Cut a piece of copper wire a bit over 1/4 wavelength at the frequency you're most concerned with. For the USA 2-meter band, that's around 19.5 inches long, but cut it an inch or so longer.
At one end, strip off the insulation and wrap it around the ground portion of the antenna connector. If it's stranded wire, you might want to tin it lightly so the strands don't separate. Exactly how you attach it to the antenna connector depends on the radio/antenna geometry at the connector and your own ingenuity.
At the other end, attach a weight - just a few grams. Enough to keep the wire reasonably straight when you're using the radio.
That's it. This gives the antenna a more constant counterpoise than your body, making it a more effective half-wave vertical dipole. You should find your radiated signal dramatically improved.
Succes, en veel plezier!