Be Professional
It's often been said that everything about freelance work is professional except the pay. Meaning, that despite initially not making a lot, you still have to act like your clients are almost over paying you.
This may seem repugnant to some of you, but another old saying is that you catch more flies with honey than with... something nasty and un-honey like. Be good to people and they'll be good to you (hopefully).
Do Good Work
This might be even more valuable than being professional. After all, at the end of the day it's about the quality of the product or service you provide. All the promptly returned emails in the world aren't going to make up for a product that doesn't meet specifications.
Ultimately, your product or your service is your leverage. When you start to feel like you aren't being paid enough and finally need to have that nice discussion with your employer or client, your quality of work should be exhibit A. It's just another way that you can make your work from home employment a lucrative enterprise.
Offer Complimentary Services Once in a While
This may seem antithetical to increase your earnings for your work at home business, but you'd be amazed at what a little extra work for free will do for a professional relationship. And at the end of the day that's really how you make more money: by investing in relationships.
When you rework a project for free despite your client or employer reneging on the original specifications, think of it as an investment in future wages. It can be another bargaining chip the next time your pay is discussed. Are any of these sure fire? No. But if you ask any successful home based worker how to increase earnings, undoubtedly a mix of these will be in there. Not to mention, that these are just good business practices.