Traffic is awful around here. Skytrain really is the best way to get around this part of town.
Regular buses: 2,25,40,48
A/C buses: 1,2,8,11,13
Phloenchit is just on the west side of the expressway and is covenient for the low sois.
Na Na is in the centre of this section of Sukhumvit.
Asok at the eastern end of this stretch of Sukhumvit - the closest stop for the Siam Society.
Another problem area for dodgy tuk tuks, particularly around the top of Soi Nana. If you're walking also be wary of pickpockets around Soi Nana (particularly behind the police box!) and also around Sois 7-9.