I would suggest that your bike is not set up correctly and your centre of gravity is too far back.
The first thing to consider is what is the predominant terrain you are riding?
- If its mostly flat consider using a technique such as alex has
suggested in his answer to cover pinch climbs and small hills.
- If your doing a lot of climbing (sitting on the front of your seat
is uncomfortable and more suited to mtb) the second thing is to
change the set up of your bike to fit you better. One of the biggest
misconceptions made by bike stores when they set a bike up for
people seems to be that they believe the rider is going to spend
their life riding around parking lots. Thankfully this isn't true,
but it does mean that many people have their bike fitted for flat
land rather than climbing.
There are things you can try at home but I would recommend talking to a professional to get their expert advice on the correct fit for your style and terrain.
- slide your seat forward in the rails (you may need to adjust the
height also for your leg length)
- reconfigure the spacers on your stem (move the spacers above the stem
and see if that has an effect)
- experiment with different stem lengths (this usually requires you to
buy or borrow many stems to get the right one).
Many people spend thousands on a bike then scrimp on a $100 fitting.