Well, if we are talking about more elaborate trips than a short day trip to a local spot, what I do will depend on where I am going. Also, this is obviously on top of all the normal travel stuff (car rental, finding hotels, etc)
If it's in the USA/Canada, I'll typically try to buy of a bird finding book for the area, check out the local listserves, and make a target list based on ebird. Although at this point I the number of ABA targets I have is small enough that I pretty much know what to zero in on. I'll also check mammalwatching.com and look for herping info if its relevant to my destination. If coastal, I will also see if there are any pelagics, whale-watches, or equivalent trips available. I generally don't do much studying, beyond looking over my Sibley and Nat Geo.
If its a foreign trip, I'll try to find relevant trip reports. Depending on the purpose of the trip (dedicated birding vs some sort of business trip where I sneak birding in), I'll look into local guides, either by checking ebird and top ebirders for a given area or just a good old fashioned google search. If its a dedicated birding trip, I might also look into either package deals through local ecolodges or see if a tour company is visiting the area at a time of year and for a price that makes sense for me. Even if I ultimately don't do a tour, it at least gives me a baseline and helps with planning. I'll also look into mammalwatching.com for any mammal info. Usually I do a lot more pre-trip studying, especially for tropical locals which have a high diversity of birds and bird groups I am not familiar. I'm not going to Ecuador until June but I am still browsing relevant field guides right now and creating lists of likely new species I might see.
Generally, I am not the sort who is really a do it yourself person when it comes to international travel. I'm find going it alone most places in the US and Canada, but not so much elsewhere. Driving even in my own country sometimes makes me anxious, never mind navigating all the aspects of travel that need to be dealt with in a foreign culture that probably doesn't speak my language. I can relax and enjoy the trip a lot more if I let someone deal with all that, and I can focus on just seeing the birds. Plus the way I figure the world is big enough and I am old enough that this might be my only chance to visit some places and see some birds. With a guide, I can maximize my species count.