'Do a little bit every day' is what Brian Cameron said. Good advice in anyone's book. Yesterday saw me use up the entire stock of fencing oddments plus some I'd nicked from Nigel and complete the full 7' length of layout on top of the wall. Mainly Ratio spearpoint but as that ran out a few odd bits of other stuff as well. That done, on to the fluffy stuff.
I'm not a convert to the newer materials like standy-up grass- if you could still get PECO dyed sawdust I'd probably be using it. Hence the bushy stuff (and there's quite a bit to do) uses what we used to call 'lichin'; the green rubberised moss. I bought a huge carrier bag from a florists a few years back and I'm still working through it. By far the cheaper way to buy it. This will, as I work along the layout, be coated with various ground foams with PVA and finished with cheap hairspray.
Once that's done I could almost get away with saying it's finished - though it isn't.