Here is a table of exposure times I use for photos of the moon/planets above the twilight horizon.

In the row with your film speed (ISO), find the f-stop you are using; follow that column down to get the exposure time in seconds. For example, with ISO 100 film, an aperture of f/4, shooting 40 minutes after sunset, the exposure is 4 seconds.

Print out this table and take it into the field. Notice that the light level changes by one stop every five minutes, so prepare ahead of time. Pencil in the actual clock times for the present day in the "Minutes after sunset or before sunrise" column. To generate a list of the exact time of sunset and sunrise for your location for every day this year, go to the U.S. Naval Observatory site.

Shoot at the best time. I have found that the horizon colors and lighting are best between 40 and 50 minutes after sunset or before sunrise, so that is when I try to shoot.

Bracket your exposures. There are other factors that affect the exposure (such as focal length, atmospheric conditions, season, latitude, film reciprocity characteristics, and the aesthetic effect you are trying to capture), so be sure to bracket -- that is, shoot extra frames a stop or two above and below these baseline exposures. These exposures were calculated for film -- digital cameras may require less exposure because they do not have the reciprocity failure that film has at low light levels.

Keep your exposure times short. If you don't want any visible trailing of the planet images (due to the earth's rotation), keep your exposure times less than:

Keep complete notes. Record all your exposures, note which photo turns out best, and use that exposure as your new baseline next time.