sharpness problem can arise in 2 scenarios
1. out of focus due to wrong focus
in this case it's not really a problem of sharpness. the problem comes with the plane of focus
lying in the unintended area. it can be due to weakness in focussing skill, meaning you can't
focus correctly, or front/back focussing from technical misalignment in the lens/camera
body.
the former can only be solved with better technique via practise. the latter can be checked
by setting the camera on a tripod and using a ruler/rule-marked object as a testing subject.
if a problem is detected you should send your equipment for servicing.
2. 'handshake' blur
this is due to instability of the camera when taking the picture. a widely accepted solution is
to take a picture at shutter speed = 1/focal length when handheld. for example if you're
shooting at 50mm or with a 50mm prime lens, your shutter speed is no slower than 1/50 sec.
however there are cases where hands are so unstable that even at 1/focal length
shutterspeed cannot eliminate the problem. and there are those who are well trained enough
to handle slow shutterspeed without the instability.
when shutterspeed becomes so slow that blur from instability is unavoidable, you can
a. open up the aperture (meaning lower aperture value, e.g. f3.5, f2.8) opening up the
aperture allows more light to be recorded on the sensor. the camera always seek to achieve
a balanced exposure (with respect to the metering), therefore when aperture is wide and
more light comes in, the shutterspeed can be faster.
b. increase the sensitivity of the sensor by increasing the value of iso setting. with higher
sensor sensitivity, the camera is able to make an image capture with lesser light entering the
camera, thus allowing you to increase the shutterspeed. high iso is accompanied by higher
amount of digital noise.
if the aperture is already the widest desired and iso is already the highest desired,
c. set the camera on a tripod, and maybe use a remote shutter release remote control, so
that the camera can be stable however slow the shutterspeed may need to be.
unsharp mask is a digital processing that duplicates and offsets the image by a tiny bit so
that the lines in the image becomes more pronounced, therefore enhancing visible sharpness.
i don't use Photoshop Elements so i don't know how it is applied in that software.
if you're using Photoshop CS2 or earlier versions, it is located under 'Filter > Sharpen > Unsharp Mask...'