I build quite a lot of dry stone walls (dsw), mainly to retain raised beds.
It is difficult to build a dsw to any great height without it becoming unstable. Stability is also affected by the shape of the stones - flat stones being easier to build with than round, river bed stones. My locallly available stone is the round river bed type so I have to be carefull. It is much easier to have the wall sloping back slightly into something such as a bank or the soil of a raised bed. If the wall is to be completly free standing then it will need to be broad at the base, eg two stones wide, and tapering in towards the top. Be prepared to build a section of the wall and then have to take it down again because it isn't right. I have to do this all the time.
Just go and get stuck in and have some fun :-)

Thankyou, fantastic tips and so speedy.

Our dry-st waller says that the stones need to be angled outwards, so any water drains out of the wall not into the middle, where it causes havoc with the infill material.

Bulba's walls are primarily for raised beds or a double dyke but all DSW's I've seen were broader at the bottom than the top. B's father used to create in his garden in Yorkshire and taught both B and our son how to do...

It can be hard work,but rewarding.Golden rule have the stone near but not in the way of were you intend to build.Start with the bulky stone at lower level,dont worry about levels (but try to put a length of string line across for a guide).you can "wedge "levels up with small pieces,If its a raised bed build you could hide any odd shapes in the wall middle,have it slightly leaning in and backfill as you progress the courses,you can put any odd shapes to the inner side.If its a freestanding wall put key pieces the width of the whole wall for stability(you could secretly layer a little sand/cement mix at various places in the wall as it goes up.For anything of height youll need to build it in sand/cement.

How high is high Chega in Scotland they build drystone walls a god 6 foot in height - no sand cement used at all!

I've recently built a "Cornish hedge" which is a thick freestanding double drystone wall with earth in the middle. Make it wide at the bottom, pack behind the stones with earth as you go, make it taper as you build upwards. Mine is about 4' 6" tall, a bit higher on the other side as it's on a slope.

Just start Millie - it's good fun, and as Bulba says, you can just take down and rebuild any bits that don't work out right. Think of it as a 3-D jigsaw with no single right answer.

That's the spirit Beattie!

Wow thanks everybody, great advice as usual. Him outdoors will have a go, I will make the tea!!!!!

Many of our walls are very similar to Beatties' but without capping stones. They are filled with compost and we call them raised beds to grow rockery plants in.

I've uploaded a picture here -
http://www.growsonyou.com/photo/slideshow/196120-i-made-all-this-patio-paths-wall/member/beattie

The band of stones near the top are purely a decorative feature. I found some large rocks of a different colour and structure to most, so saved them to put at the top, for no particular reason.