Ok. You are all of twenty three. Smart, hip and happening and life would be perfect if you could only go onsite. Good. But you are not alone. Every person at your stage in life wants to go onsite. As a manager I can see for myself the eagerness young people have in this regard and demands to go onsite are commonplace during performance appraisals. So here's the magic formula. Err…sorry there is no magic formula but these thoughts can at least (hopefully) let you know if you are on course.
Align: The most important factor to go onsite is the existence of the opportunity. You can get stuck offshore for years (collecting performance awards :-)) if you ain't in the 'right' account. Look at the current onsite-offshore ratio. That should give you some idea of the past trend. If you are in an pure offshore account, see if you can speak to your managers and get a switch. Do not state the lack of onsite opportunities as the reason. You are likely to be denied. Look for more logical reasons.
Perform: Onsite opportunities are often used as an incentive to retain top performers. Before you make your case you need to be an above average performer-at least. Ensure that you get decent performance appraisals. Remember that at onsite you will not have too many people to help you when you run into trouble.
Communicate: Matthew 7:7: "Ask, and it will be given to you; seek, and you will find; knock, and it will be opened to you." If you need something you need to ask for it. But just plain crying never got anyone anything except when they were in diapers. Ranting does work, but provided you are a star performer. 
Identify skill gaps: Ask your boss what it takes to go onsite and specifically ask him what is it that you need to do. Identify skill gaps if any and address the same. Here are some skills that are hot from an onsite perspective (at the Sr.developer level):
   -Build and deploy skills
   -Fast bug fixing and problem solving skills (UAT kind of situations)
   -UX skills (prototyping etc)
   -Requirements gathering and analysis skills.
   -System testing (esp for mainframe projects) 
Plan the logistics: Ensure you don't run into any logistical hiccups like expired passports or other personal problems.

Good luck and if the if the above tips work I need a thank you mail from onsite! :-)