Some boxes go missing because they are poorly hidden in the first
place. Read the Faqs about good hiding spots. Refer to Randy Hall's
Letterboxer's Companion, especially pages 28 - 31. Find a few first
if possible- actually, more than a few. Another recommendation to
beginners is not to place your first few boxes in an urban setting.

Hiding Boxes from the LBNA FAQs
In the field, locate the specific hiding place for your box and work
backwards. Find the hiding place first because good hiding places are
difficult to find, but clues and directions can be written in pretty
much any circumstances. If you start off at the "start" and write
clues, you may end up in an area where there is no suitable place to
hide the thing. The goal is to make sure someone without the clues
has a zero chance of discovering it accidentally. It is important
that only people who understand what letterboxing is find your boxes.

Hiding it under rocks and natural rock ledges seems to work the best,
followed by downed, hollow logs. Less ideal are makeshift piles of
sticks and bark, but that will do in a pinch, if the location is
protected from wind and storm water run-off. I recommend against
burying the thing, because you don't want people digging in parks,
many of which have artifacts buried and restrictions against digging -
- such activity will give the hobby a bad name. In Dartmoor, they use
a technique called "plugging", which the moorland terrain is suited
for. I do not recommend this in the States unless you and the people
who have your clues are familiar with the technique, and the terrain
lends itself to such.
I recommend hiding the thing in areas where there are rarely any
passerby. Boxes that are well off-trail are more likely to not be
accidently discovered, but they run the risk of having a social trail
develop to them if they have alot of visitors. Some parks require
that any boxes placed be right on the trail to prevent this, so know
the area and the rules first. Consider how many people will be in the
area of the box during high season. Make sure wildlife cannot move
the thing (but I am personally against tying it down, for fear of
wildlife getting tangled). When hiding it in grass or bushes,
consider what the vegetation will be like in the dead of winter (i.e.
gone possibly). I made the mistake of hiding my first box under a log
in thick grass in the summer, only do discover the grass completely
gone in winter, and the box visible from 50 meters away!