Bakerloo to Baker St, Jubilee to Swiss Cottage, 268 bus to Rosslyn Hill,
short walk to Hampstead Heath.

I do this journey quite frequently (from Paddington to Hampstead
Heath).

Best route I found was the one aboe but it is better than implied.
From Swiss Cottage, you can either get the 268 (with the short walk at
the end), but you can also get the 46 or C11 (both of which go to
Hampstead Heath NLL - or pretty near, anyway). Between these 3routes,
you rarely have to wait more than 3 or 4 mins.

The slight snag can be seen in this map:

http://www.tfl.gov.uk/tfl/gettingaro...ttage-2249.pdf

Stop B has buses C11and 268, Stop F has buses 268 and 46. In order to
maximise your chances of getting any of those you have to stand near B
and sprint to F if you see a 46 coming up through the Swiss Cottage
one way system. A lot of people do this, so you won't look odd