Is this going to be a Company sponsored trip? If you're going to take vacation to go to the conference and pay your own way then it shouldn't be hard to convince them. The time off would allow you to re-charge your batteries in a relaxed atmosphere and nothing enhances one's ability to learn than a relaxed atmosphere (especially if it's doing what you want). If your trying to convince management to pay your way then I have to agree with the others on this. In Manager speak how does this make him/her (your Manager) money? In some way you have to show that if they spend $xxx.xx sending you, they will save 2X number of dollars later in the year or they will make 2X number of dollars (use the latter, in Manager speak making money is better than saving it). Managers love options also, try to give him/her some options. Try to put it in terms like "it would cost $xxxx.xx to send me to XY school, it'll take ZZZ hours of reading books and doing the old test and see method, or it'll cost $xxx.xx for me to attend this conference which I'm absolutely sure I'll learn more stuff than you'll ever know about". Stay away from the social aspects and the ME factors. If all your Manager is hearing is "I want to go..." and "it'll do me so much good..." then you're not going to go. Try to show them that "it'll be great for the Dept...", remember they (management) simply want to look good and make money. If you put it in their terms it should be more convincing.
HTH