Been doing services and sales on JSP on and off since I created my account through WoW, D2, POE, D3 and more.
1) Recognize your value. Yes, jobs pay well but you need to walk before you can run. Push for competitive pricing for your "worth" on the site. Sometimes this means doing jobs for less FG/gold, typically aimed at more veteran members, because legitimate vouches by veteran / trusted users are worth their weight in gold. Especially if they carry a Trusted/Mediator/Moderator/Former Moderator tag.
2) The 2/3rds rule. 1: Posts, 2: Join Date, 3: FG. A lot of users trade only using the 2/3 rule. Whoever has more of 2/3 of those things can be seen as the more "trusted" one. This isn't always the case, but it's a good rule to follow. How do you make use of this? Be an active member of the community first and foremost, and build up your profile through time. It took me a long time to feel like I was a legitimate member of the community. The best time to start was in the past, but the second best time to start is now.
3) Legitimatize your profile. Your profile has no avatar, no signature, no guild, it's blank. When someone looks at it, they see standard JSP blue and white with nothing to personalize it. You need to humanize yourself a little bit. Show some effort.
4) Like others have said, jobs with little risk is the most important aspect of starting out in services. Some people will work with you, because they want to see the community flourish. They will not, however, be completely reckless and risk everything so easily.
5) Read the rules. Your warn level is public. Your warnings are not. This means that having a warn level can make you be seen as someone incapable of following rules. Would you let someone incapable of following rules have access to something you spent hundreds or thousands of hours on?
6) Communicate. Engage with the community as much as possible, engage with people you work for as much as possible. Post a thread of your progress on your RS main. If you play other games, post progress of those there. Put the link to those types of things in your signature / notes section of your profile.
Some of this has already been said or implied in different ways, but it's a few things I took into consideration when establishing myself on the site, and a few suggestions I have for everyone.