There are 3 main variables that need to be considered for effective imaging session planning:
It's fairly difficult to change the weather, so planning is really restricted to deciding whether or not the night in question will be suitable for the intended target. In the Wagner Valley where I'm located, there are very few nights that have widely separated cumulus type clouds - it's usually clear or completely socked in. I do use Clear Sky Clocks to estimate the transparency and seeing for 1 or 2 nights ahead, and find it to be usually fairly accurate. I also depend on the Wunderground weather site, especially the Scientific Forecaster Discussion and the Satellite images. The Scientific Forecaster Discussion gives you a pretty good idea of the confidence in the models, and by animating the Satellite IR and Visible images, you can visualize the flow of any clouds and high overcast.
Note that Transparency and Seeing are two completely different things. From the Clear Sky Clock website:
The line, labeled Transparency, forecasts the transparency of the air. Here 'transparency' means just what astronomers mean by the word: the total transparency of the atmosphere from ground to space. It's calculated from the total amount of water vapor in the air. It is somewhat independant of the cloud cover forecast in that there can be isolated clouds in a transparent air mass, and poor transparency can occur when there is very little cloud.
Above average transparency is necessary for good observation of low contrast objects like galaxies and nebulae. However, open clusters and planetary nebulae are quite observable in below average transparency. Large globulars and planets can be observed in poor transparency.
The line, labeled Seeing, forecasts astronomical seeing. (It's an experimental forecast.) Excellent seeing means at high magnification you will see fine detail on planets. In bad seeing, planets might look like they are under a layer of rippling water and show little detail at any magnification, but the view of galaxies is probably undiminished. Bad seeing is caused by turbulence combined with temperature differences in the atmosphere. This forecast attempts to predict turbulence and temperature differences that affect seeing for all altitudes.
Bad seeing can occur during perfectly clear weather. Often good seeing occurs during poor transparency. It's because seeing is not very related to the water vapor content of the air.
It's also fairly difficult to control the phase of the moon, but there are a few useful tactics to deal with it.
My choice of imaging target for a particular night used to be fairly random - I'd use Cartes du Ciel to look at the position of a few Messier objects, and choose one that had a fairly high elevation, and was away from the Medford sky glow to my North. This worked OK, especially when I was just getting started, and just about everything was new to me.
Now, however, I plan a lot more carefully. I start by keeping a list in SkyTools Pro of potential targets. I gather these from various sources, but mainly the various Astro Yahoo Groups, Cloudy Nights, and a great little book titled "The 100 Best Astrophotography Targets" by Ruben Kier. In SkyTools Pro I add the object, a link to any pictures on the Web to give me an idea of size and color, and a link to anyone's webpage who I think has done a good job on the target. In particular, if the web page gives the equipment and exposure details, that helps a lot with session planning - it provides a good starting point.
Of course, the choice of target really depends on the time of year. The main criteria for me include:
SkyTools Pro does a great job of making it easy to meet all these criteria, and more. Once you've set up your observing location, elevation, estimated sky darkness, telescope, and camera, it calculates a Quality rating for every object in your list, and for the selected object, desplays a handly little graph that shows object elevation and quality throughout the night.
SkyTools does a lot more - once you've chosen a target, you can view it in the Interactive Atlas, and turn on the Context Viewer, which displays your camera field of view as well as your guider FOV. This makes it easy to fine tune your framing. I now find SkyTools to be an indispensable part of my toolbox - highly recommended.