How have your beers turned out when you add the extras? This is
something that I have tried with a semi-success....

I also use kits and tried boiling up my water with a variety of spices
(cinamon, mixed spices etc) in a large cotton teabag like way. Once the
water had cooled, I added this to the wort. Once the beer was ready to
drink it tasted fantastic and really christmasy, justthe way that I
wanted.

WHile over the last couple of months I have been 'sampling' this beer
(over half the batch gone now) I have noticed that the great taste of
christmas appears to be dissappearing :( Although I haven't tasted any
for about 3 weeks now, I do know that it will no longer have any of the
fantastic flavour thatI brewed it for :(

How have you added the extras and have the flavours lasted at all?

I did have the same problem with a stout, except in this instance, when
kegging, along with the priming sugar, I poured in a bottle of Ginger
Wine. Again for the first couple of weeks this had a fantastic ginger
flavour to it which slowly faded over time. The only thing that has had
any lasting effect was a stoutt hat I added a few pots of freshly
brewed coffee.....