'locking'selection or objects
 

is there any way I can isolate an object so that I can't accidentally select it whilst working on something close by?

sure. You should use a layer. Select an object & make a layer with it. On the layer panel choose the second box & swich it 2 times.
It can be Empthy / template / reference

Swich to the reference & you cant touch it.

you could put it on a layer and set the layer to template. or you can select the shape node of the object and under object display check template there.

I forget to mention the Isolate button.
Its very handy one.
Choose the object you want to work with & push the blue wireframe cube & go on :)

thanks but - pushing that just hides the other objects in my version - i need to see the obect, i just want to avoid selecting it! :\

If you want to see objects in the scene but avoid selecting them then layers are you best solution. Place the objects in a layer then you can make that layer either a reference or a template layer.

Switching to camera based selection (hold down the q-key and left mouse button will bring up a marking menu) can also be a quick fix for those cases when you are selecting a small object in front of a larger one but maya insists on selecting the bigger object.

hmm layers.. i'm having a bit of difficulty using them in 2011 student - the little check boxes (to select visible,template etc) seem to have a life of their own. sometimes they work and sometimes not with letters appearing at random??