Guidelines for trainees who request BEST funding support:
Funders want to know the impact their investment will have and that you will not be the only one to benefit. Can you offer to teach others what you learn on your return? Is this of interest to a larger community? Will you work together with others who have also shown an interest? When you send a request that sounds like it's a nice to have as opposed to a ‘can’t survive without it', it gives an easier way for supporters to say no.
Have you looked into competitive offerings? (Are there even any?) If you don't mention this, one might think: how is this one so much better than something on campus, or within a 3h drive? Have you already made use of local offerings (classes, events, etc) that provide some background knowledge?
Are you willing to find your own free housing or carpool with others? Have you contacted the organizers to inquire whether they have sponsorships for students, or at least discounts? How about the sponsors listed, would they consider a request? If they have all said no, or at least you are intending to ask them, then it is more clear you have explored other avenues and exhausted other possibilities.
Don't make anyone do any work to find out how much you are asking for. Don’t make them wonder what you will get out of it, how you justify the value and why it's absolutely essential for your future success. Have a budget spreadsheet on the total cost (airfare? meals? hotel? ground transport? registration? what's included? what are you paying for? who else has committed to support you?). Like with any investment, no one wants to take all the risk. They want to know you have some skin in the game.
Also, do include a deadline for a response, especially the deadline for early registration, for example, and send it in a timely fashion so reviewers have a reasonable time frame within which to work.