For a remix, I've read some people ask to be paid up-front few hundred dollars without royalties after that, and some ask for a % of the sells.
Personnaly, I had something like that :
"REMIXER agrees to give exclusive rights to LICENSEE. Compensation will
be paid at a rate of fifty percent (50%) of royalties earned by ARTIST as stated in ARTIST contract. Payment will be made in accordance to Accounting statement.
Royalties are delivered two (2) Terms Yearly – delivered by means agreed upon by LICENSEE and REMIXER in appendix “information sheet” – no later then 30 days after January and July 1st each year when balance of Royalties exceeds $50. If balance is below, LICENSEE will hold onto royalties until balance is above such value. If four (4) Consecutive terms pass where balance remains below $50, remaining royalties are returned to LICENSEE and account is closed."
So %50 of what the artist earns, and I get the money if my balance goes above $50...
Well I had this few times, and I actually never earn anything from remixes that I did. Sweet. Although I'm pretty sure the sells were extremely low so I don't blame anyone, I wonder how can I even check if it's dodgy or not.
So how do you do guys ? isn't it better to be paid upfront ? it seems a better way to earn some money.
Anyway, discuss about it, give advices, experience, how much do you ask for, upfront, percentage , how do you check, etc ... share with the forum !!
so I guess I cannot ask for upfront. 
That said, if a track sells 4 copies, I still get my upfront fee... so it's a gamble. However, I do get a ton of publicity and more work because of the successful ones.