You kind of have three options (I've ranked them from best to worst):
1) Either in vest in a pair of studio monitors/studio monitor headphones
2) Listen to mixes similar to the one you're trying to create on your headphones or speakers (one of the two) and try to make it sound similar
3)Listen to all your music for the next couple months on your headphones, so you know how good mixes sound on them and you can try to recreate that. Also learn how they're eq'd so you know what frequencies your headphones have boosted, and boost them yourself, and lower the volume in areas where your headphone has bad frequency response (i know that sounds counter intuitive but there's good reason for it (I can explain if you want))
4) Find a bunch of headphones you have, borrow, steal from friends, whatever, just get as many as you can, and tweak the mix until it sounds good on all of them