To make sure that the agent is going to look favorably on your next book, I usually write the synopsis and first three chapters and send them to him and ask if this is something that he might be enthusiastic about. In other words, it has to ring his bell. I do this as a precautionary measure. My agent will take many things into consideration--if there is a market for it, if it has a good concept/premise--that all important hook, and so on. You don't really have to send the first three, but I would run the idea by him and ask for a yea or nay. Agree to hit on something that gives both of you the happy shivers.
There is nothing more devastating than writing full tilt on something, spending six months, and then having your agent tell you that it isn't going to fly. Your agent has to fall in love with it too. I had this happen to me. Fortunately it was a trunk novel that I brought out of moth balls and reworked. He still did not like it. That's one that didn't get through the gate.
Hint: Ask your agent what's hot out there. What are they looking for? What stands the best chance of being picked up? Where are the open slots? If he/she has an idea or solid opinion, and if it falls within your genre parameter, and you've got the voice/style to carry it off, go with it. You don't have to do this--it just ups your odds. I'm not saying to hack it and write outside of your box. I'm saying to discuss a genre switch with your agent. They have their pulse on the trends of the marketplace. Sorry, I'm not a real fan of writing whatever the hell I damn well feel like. I'm not out to entertain me; I'm out to entertain the reader--all of them I can get. I know what you've been told by your peers, but I think that will work for you in an agent-less situation. I'm sure you've heard, "Write what you love." This can work very well, since these books are often written with the most passion and purpose. But if I've been doing that for three years without success and my agent suggests I switch genre gears, then I'm going to listen. I always thought I was a capable science fiction writer until several agents and editors told me I excelled at action/adventure. So now I just put a lot of action in my paranormal romance and thrillers.
As far as agents dumping clients. I don't see a lot of that happening. What I see more is writers getting frustrated with their agents and dumping them. Case in point: I've been with my agent for 22 months. From the beginning, he had 29 fiction writers in his stable. Today he has 25. The evidence shows that three of those writers left him. He dropped the other client for personal reasons. He also represents a dead author, who started out with him at the very beginning. He made a vow that he would continue to champion that book out of respect to the estate. And, of course, he's wild about the book. Now, THAT'S diehard service going above and beyond the call of agenting.
You're going to get genre "niched" whether you like it or not. I don't even think that's a word. Your maximum sales potential will come from a fan base that is launched from your first book. If it does even respectively well, you'll be encouraged to stay there, for strictly marketing considerations. It will do you no good to write all over the genre topography and hope that one of those books hit, unless you're switching genre gears. Your agent will know where your strengths lie and tell you so. You have only to ask. When your last name is King or Koontz, you can play all over the sandbox if you like. Until then, try, try to specialize.
At the behest of my agent, I dropped science fiction. The market was just too tight for us to make a debut splash. I had that same problem when I was repped by another agent 18 years ago. So I slid rather comfortably into paranormal thriller/romance and urban fantasy. The result has been that I'm getting about 35% more full reads in that category. My agent was right. I upped the odds. The transition wasn't hard--I love words anyway, and it's still spec fiction. I'm strong there--I feel it and I know it.
Don't irritate your agent with trivial or non-business type communications. Unless you're really best buds. Successful agents are REALY BUSY PEOPLE. I check in about every three weeks--sometimes a month. That's unless you're doing rewrites or hammering out a contract. New repped writers have a tendency to suffocate their agents with needless questions and pestering. Give 'em some breathing room, and everything should fall into place. Contrary to popular belief, your agent hasn't forgotten about you. They are really dying to send you that all important publishing offer-contract. You know the one you've been waiting for all your life?