First thing you should do is not think about it as Part 1 and Part 2 of a cliffhanger, but one complete story presented with a very extended intermission. As the writer, you should know the answer to every possible fan question that will be asked in between the first run on the cliffhanger.
I consider two well known and quality cliffhangers to be "Star Wars V: Empire Strikes Back" and "Star Trek The Next Generation" episode "Best of Both Worlds: Part One" and both are night and day in the execution of what I will term the Short Stop and the Sudden Stop (and yes, I am paraphrasing from Pirates of the Caribbean)
In Star Wars, the drop is subtle and leaves a lot of lingering questions to the viewer (Is Vader Telling the Truth? How do they save Han? How are they going to stop the Empire? Who is the "other" that Obi-Wan spoke about?). Notably all matters directly tied to the conflict of the film are resolved and while the Empire won today, the heroes are all alive to fight tomorrow and we have some new guys to boot. Our heroes are broken, beaten, tired, battered, bruised, injured, maimed and/or mangled, or rendered artistic conversation pieces for the time being, they are going to be around for the final chapter. The final ending scene is bitter sweet... we're going to set things right, but we need to regroup and breathe a bit. This is merely a Short Stop.
Best of Both Worlds on the other hand is designed to build up to the very last line of dialog. Every moment of the episode slow builds from there... the opening is like any other ordinary episode, but by the final moments the tension is ramped up so high that the viewer might not realize a cliff hanger is coming until the tell tale "To Be Continued..." is played on screen and one slightly more aware of his surroundings has checked his watched and relized they have minutes to cover what would be 5 minutes of drama AND we haven't the coda to end on. When the pre-asskicking one liner is finally given (Ryker: Mr. Worf, Fire!) the camera work, combined with the dramatic musical build up tell us right away to pay attention to this moment, before cutting to black with the Cliffhanger's signature promise. Naturally "Part 2" opens with Worf pressing the fire button which not only fires the super-duper awesome gun but also completely changes the carpet pattern in the blink of an eye, but picks off right were the cliffhanger ends... and if viewed as originally aired, is a 3 month real world gap between the order given and the attack and the results. Here, rather than containing the story within itself, the story is given the feeling that the first 5 minutes of Part 2 could have been fitted into the tail end of Part 1 and made Part 1 a short stop cliffhanger or even an entire episode. This is a sudden drop.
Both can be used to great effect and both don't have to write the conclusion before you release one of the books if necessary, but it's critical to know what happens next as you write it. Know where you want to go.
I say this because the reason for Best of Both Worlds meant that they couldn't write the conlcusion. It was the season 3 finale and Patrick Stewart's contract was up and there was some concern he was not going to renew. Both Worlds was written with the idea his character might not be returning so the set up was for in case he didn't return to have his character exit and the script would be written and shot over the summer when they had better knowledge of what's next. Subsequent Season Finale Cliffhangers were of mixed success as Both Worlds and even then, the Part 2 is not as well loved as Part 1.
Have your finisher some what fully formed when you end and you can avoid this somewhat.