OK, well I rarely use lamb mince, I am more a cottage pie person but to this I add (depending on how the mood takes me) tomato puree, or wooster sauce (cant remember the correct spelling), or bisto granules, or chillies or anything else i fancy. Not so sure these would work with lamb mince though. Perhaps you could try some garlic or rosemary which both go well with lamb.
Also, again this is for beef mince, the mince should be cooked long and slowly, not the 20-30 minutes as usually stated on recipes. A good 1hr 30 mins of cooking really brings out the flavour and makes the meat more tender. I imagine the same will apply to lamb mince.
You could also flavour the mash with something, perhaps finely sliced spring onions.