how to do this transient mark trick

[matthias]

Hi there,

I have

 '(transient-mark-mode t)

in my custom-set-variables and for each of the four arrow-keys of my 
keyboard i have a function of the following kind in my .emacs file and i 
bind the according function to the corresponding arrow-key:

(defun set-mark-and-move-point-right ()
  "sets mark if mark is not set and move point one line up"
  (interactive)
  (when (not (region-active-p))
    (call-interactively 'set-mark-command)
    )
  (forward-char 1))

region-active-p looks like that

(defun region-active-p ()
  "Say whether the region is active or not."
  (and (boundp 'transient-mark-mode)
       transient-mark-mode
       (boundp 'mark-active)
       mark-active))

i achieve the desired behavior that the region is activated and 
colorized by transient-mark-mode when i press shift-<some_arrow>. 
However i have the feeling that this is not a clean way to achieve this 
since using these functions I a can also

press shift-<arrow>
release shift already
press some <arrow>s and region is still colorized


call for help is like this: Can it be done better? Is it possible to 
de-activate the region when shift is released?

thanks for your help.

matthias