Choose the languages you will learn. Your circumstances may dictate the languages you must learn--for example, a career change, an upcoming foreign trip or a university requirement. However, if you are able to choose your own languages, select languages in two different language families. The more distinct the languages, the easier it will be to keep them separate in your mind as you learn them.
Select your resources. Successful language acquisition incorporates reading, writing, listening and speaking in the target language. Choose materials and activities in both languages. Search the Internet for articles, children's books, podcasts and news broadcasts in both of the languages you are learning.
Set up a program of study, designating separate times for the study of each language. For example, if you have chosen to learn French and Chinese, take an hour and work exclusively in French. Listen, read, write and attempt to think French alone. During your Chinese hour, do the same sorts of language study activities in Chinese, thinking nothing about the French you worked on earlier. Focus on the unique aspects of the languages more than their similarities.
Focus on your goal, and don't get frustrated. As you continue to learn your languages, incorporating grammar study, listening comprehension, vocabulary memorization and conversational practice, realize that your mind may have a tendency to confuse the languages. This is normal--relax and don't give up. Keep practicing your languages consistently, and it will happen less. Your mind is capable of handling many separate "codes" and will learn to assimilate the languages successfully over time. Each subsequent foreign language will be easier to learn than the previous.
Seek out native speakers in your languages for conversational practice. Once you have gained a basic understanding of the structure of the languages you are learning and have some vocabulary under your belt, you are ready to work with a language partner. Ask for feedback and correction; this will help you to improve and move you toward mastery of your languages at a faster rate.