For me, it all depends on the number of trips I've been on, the number of days on the trip, and who is joining me on the trip. For instance, my trip with my BFF for 3 days was a lot less planned than, say, my 5-day honeymoon.
Generally, I have a small notebook, and have pages divided like this:
Page 1: Trip Specifics, with reservation codes as appropriate: Dates, hotel accommodations, air travel details, ground transportation, any meal reservations
Page 3: Hotel Comparisons: Hotel name, price for the whole trip including taxes, rooms available (ie, 2 queens, 1 king, 2 queens w/ bunk, etc), and any code for the rate (such as my corporate discount). Generally, I take all the hotels I'm willing to stay at, and hop online to check prices, and list them all on one page with all the possible information needed. Once reservation is made, it is entered on page 1.
Page 5: Air travel: Airline, airport code, price of flight each way. Once flight is paid for, information is entered on page 1.
Page 7: Ground transportation: Estimated cost of taxi, shuttle, and any other possible transportation ideas. Once decided, information is entered on page 1.
Page 9: Meal ideas, or other must do items. Any reservations made will be entered on page 1.
Page 11: Any special needs re: the trip (ie, specific clothes for Disbounding, etc).