Great job everyone keeping this civil, I hope it can remain this way.
For the record, I'm the owner of this site, a pretty normal person (Terrie may argue) and am a "Mormon" or member of the LDS church. I served a mission when I was 19 for two years in Massachusetts (from Revere and Dorchester all the way out to Pittsfield).
Missionaries volunteer, and also usually pay their own way, to spend 2 years (1.5 for women) to do full time service (no TV, no movies, no dating, etc.) to share a message with interested people who may want to learn more about what has brought these missionaries happiness. Missionaries are taught to be patient and polite and in general are (in my opinion) not as "pushy" as some may think. That being said, missionaries come from all over the world (I had one companion who came from Laos to be a missionary in the US), have very diverse backgrounds, and so there can occasionally be some missionaries that are more energetic about sharing their message than others (usually this is the new missionaries who are like a kid who just got done opening presents from Christmas and cant wait to tell all his friends).
On my mission I really didn't feel like I was trying to convert people to a church. I felt more like I was going to people who are spiritual brothers and sisters (all children of God) and finding out if they would like to know more about where they came from before they were born, why they are here on this earth, and where they are going.
For me, it was like telling people there was a really good ice cream shop in town that has super good ice cream. Some people may not like ice cream... some people may already have a favorite place to get ice cream... well, as missionaries we just really liking the ice cream at this one place and wanted to tell others about it. We do NOT want to force anyone to eat it or force them to even try it (flashback to brussel sprouts). We just look for people who may be curious and want to know more and sit down and talk.
As others have said, if you don't like ice cream, don't want to try other ice cream, then just politely tell them (as you would anybody else) that you appreciate they are doing something they believe is good and right, but you aren't really interested. On the other hand, if you are interested you can tell them you're interested in learning more about what their beliefs, purpose of life, eternal families, etc. but that, as you said here, "I'm not ready to convert." If at any point you feel like they are just there to convert you then politely say, thanks for your time, I feel I've got a good idea of what you came to share, have a great day."
As a missionary I enjoyed answering questions about the church and correcting misconceptions. I was also very happy to find people who felt lost or confused about why they were on the earth and I had the opportunity to share more information about the purpose for life. That said, many were not interested so I simply said, "Thank you for your time, have a great day."