If you are keeping the same displacement, then you can't really increase
torque without forced induction (turbo/supercharging).  O.k. you could
install nitrous, but that's an extreme option.

When most people 'tune' thier engines, they allow the engine to rev more
freely - this gives more power, but no extra torque, unless the original
intake/exhaust was very restricitive (which it isn't on the 306).

Alternatively, you can make the car lighter, by stripping out non-essential
items. By removing the spare seats/wheel/running a half full petrol tank,
you can pretty easily loose 10% of the car's weight, thus increasing the
power-to-weight ratio.  Done carefully, it's completely reversible, so the
car is still saleable, it's free and it doesn't put any extra strain on the
drivetrain, so reliability should be unaffected.

If that doesn't appeal, then from a purely pragmatic point of view, it will
almost certainly be cheaper and easier to sell your existing car and buy a
quicker one - insurance on modified cars is usually expensive and the
resale price will probably drop, so you get no return on your investment,
whereas if you spend the same buying a faster car then it will be worth
more at resale.

All the best,
Andy