Travel to a place where your target language is spoken, if you can. Hit the streets and attempt to speak to people in that language. Resort to English as little as possible in order to achieve as much immersion as you can. If you don't know a word or a phrase, work your way around it by finding some other way to explain what you mean to say and asking for the correct word or phrase.
Apply to a language-immersion program if you have the funds. Middlebury College in Vermont hosts the most famous summer language immersion academies in the world. They require nine weeks of your time and you must commit to speaking nothing but the target language. Middlebury's programs ran around $7,000 at the time of publication. If your target language is Arabic, the American University in Cairo runs the most prestigious summer Arabic program in the world.
Seek out communities in your area in which the target language is commonly spoken. If you are in the Washington, D.C., area, for example, nearby Annandale, Va., hosts Koreatown, while Seven Corners, Va., is home to Little Saigon. Go out and speak with people as much as you can and as often as you can. Decipher signs and make notes of vocabulary words. Listen to what people are saying around you.
Get a hold of newspapers in the target language or find foreign language news websites and read them every day. Use a dictionary to look up any words you don't know and write them down multiple times in order to memorize them.
Watch television stations in the target language. Turn on the closed-captioning in order to read along as you listen and pick up words and phrases.