A Slik Tripod

No, no that’s just the manufacturer’s name. This is a fairly heavy tripod but still it can move slightly when pressing the shutter.
On soft ground the tripod appears to float and lacks stability.

A simple solution is to hang your camera bag from
the centre post. (Bag should be heavy, Preferably full of beer).

If your camera is equipped with a self-timer by
all means use it for tripod photos. Either that or a shutter release cable will
keep your shaky hands away from the controls. If you can do none of these things
an alternative is to press down on the camera (pushing the tripod into the
ground) when you snap the picture.

If you are using an SLR operational vibration,
especially from mirror slap, can reduce resolution. If you can lock up the
mirror, do so.

Shutter speeds between 1/2 and 1/30 sec appear
especially vulnerable. Longer exposures minimize the effect of motion. Try 1/8
or 1/4 sec.

The lighter the camera the more susceptible it is
to vibration.

Finally when shooting outside reveal your tripods
spikes (assuming it has them) and jam the sucker into the ground.