I will be in Tokyo for a conference and was hoping to check out Kamakura for a visit.
Is it practical to travel to Kamakura for a day trip, or should I look into overnight accommodations?
I'll try to answer on the assumption that you're asking mostly about the relative practicality of a day trip, rather than asking for a sort of value judgment on the merits of an overnight.
You can see a number of popular tourist destinations on a day trip to Kamakura from Tokyo. Additionally, it's a somewhat sleepy city that, in comparison to much of Tokyo or Yokohama, as temples and shrines close to the public around sunset and retail mostly sort of shuts down around 5:30-6pm or so, and unless you're staying for an unusually late dinner and drinks sort of thing, it's perfectly easy to get back to nearly anywhere in Tokyo as long as you leave by 10pm or so.
Train frequency decreases later in the evening, and that's the main risk you have to getting back to your hotel. If you plan ahead or use the mobile version of Jorudan's Norikae web site, that shouldn't be a big problem.
The primary benefit of an overnight is that it simply isn't practical to see more than two or three of the major shrines/temples in a single day. Some of the best known ones, like the Daibutsu at Kōtoku-in, can be a pretty substantial walk from other major destinations, like Hachimangu Shrine, so if you have a long list of places you'd like to see and you're not motivated to navigate local buses or take taxis to reduce your travel time, or if it happens to be a busy tourist season or pilgrimage season with lots of lines, you might lose a lot of time to travel and waiting. If you have lots of places you want to see, and you want to navigate the little shops selling lacquerware and leather goods and so on that Kamakura is fairly well known for, allow time for lunch and snacks, and stop impromptu at whatever sights you stumble on without advance planning, it can be nice to have the advantages of an early morning after an overnight stay so that you don't have to push yourself too hard.
You can keep the travel time down to 45-50 min. one way if you plan in advance. And it depends where you are staying in Tokyo.
I think the most convenient is if you are staying around the southern part of Tokyo. For example, if you are near to or can easily get to Shinagawa station, you can take the JR Yokosuka line to Kamakura (51 min. / Y690). You can find current train schedules on Hyperdia and other websites.
Because its so close to Tokyo, even if you return to Tokyo and go back the next day, it should be easier than making a separate reservation around Kamakura.
But as JasonTrue mentioned, you should plan where you want to go in advance. Some places are hard to find, and keep in mind that you will be walking a lot. A couple of websites to check out:
Though its a way outside Kamakura itself, the Daibutsu (Great Buddha) is a must see and on most every tourists agenda.
I would also do a day-trip. I've done that a few times and it's a fantastic day out. To make the most out of your day trip, you might want to consider rentinga bicycle at the station in Kamakura. This gives you the chance to move around quicker and see much more than you would using the bus or just walking.
Here's a suggested itinerary: http://japantourist.jp/view/kamakura-by-bicyle
As for the train timetable, I find this one very useful: http://www.hyperdia.com