Yeah I only work on half of the model - but I also use CPS - it's a poly modelling thing which basically creates a smooth duplicate of your model - you work on the low poly part and the smooth part updates with your changes - so you can see how it looks smooth - it's a very versatile plugin!!

Merging verts - after you have duplicated your half of the model - slelect both halves then 'Combine' them - this should merge all the verts but rarely does - select all the verts down the centre line - then click on 'Edit polygon' then 'Merge vertices' - this should merge all unmerged verts

Again a basic explanation as there are times when this also doesn't work i.e. if your verts are not exactly down the centre line or are to far apart to merge.

Hope this helps some.