United Nations: How do I get a job at the UNDP (or any other UN/agency)?

By using http://www.unjobfinder.org/ you can search for all current vacancies in all UN organizations. They also have the function so that you can target your search to match your interest and experience. By following their blog, you can also get different advice on the application process, levels, etc.

See if you can connect with anyone working for an agency you're interested in, the UN is HUGE (with multiple agencies/programs that might not even interact with each other at times). Pick that person's brain and see how you can use the next few years to prepare yourself. Otherwise, continue working in the job you're doing (if you're enjoying it). Unless you have very solid connections, getting a job with the UN is very difficult (it's one of the most stable and highest paying jobs you'll find in a developing country - as a result competition is fierce). Instead of trying to get it in the short term - think of what you need to do to prepare for it in the future. Look up the requirements of a position you want and see how you can start preparing for it now (a permanent UN job, even official internships, requires a master's degree). While you are working now, see if you can connect with or work for an NGO or other organization that does a lot of work with a UN agency you want to work for so you can build your network and follow up later. Also, start brushing up on your language skills (English + French, Spanish, Russian or Chinese).