Is there any thing I can do to keep the clematis in
bloom during the summer?

Probably not. Or, maybe more accurately, if you *do*
find a way to keep it in bloom all summer long (at
least with a lot of blooms, mine seemed to have
occasional blooms on them most of the summer), you'll
be risking weakening the plant, maybe to the point
where it will die over the winter and/or risking
*next* year's blooms.

I'm not an expert and will defer to any dissenting
opinions, but, from what I've read on clematises,
they are generally 'certain parts of the year' only
bloomers. Some bloom in the late spring/early summer
and spend the rest of the summer putting out new
growth for next year's blooms. Some bloom in the late
summer, using the new growth from spring to their
bloomtime to put out their blooms. There's another
period or two where certain clematises will bloom,
IIRC, but it seems to me that I haven't read of
clematises that bloom the entire growing season.

Tracey