Yup - i have no access to the inside of the lens (even though i swear i can see some dust particles i would love to get rid of in there) so have to be content with only cleaning the outside.
I personally use my friends 'Lens Cleaner Pen' - he is an amateur photographer who has a 'pen' that is a brush on one side (to sweep the dust away) and a small cleaning sponge on the other end (to get rid of any stubborn marks)....similar to this:
Lens Cleaning Pen for DC DV SLR DSLR Lens Filter UV CPL on eBay (end time 24-Oct-09 03:45:00 BST)
Nice and cheap - cleans the lens well and doesnt cause any damage.