Learning a small, local language like Gaelic is fundamentally different from learning a massive, international language like French or Spanish. Most people come to grief when they try to learn Gaelic because they approach it like French or Spanish, and you just can't learn Gaelic that way.
Tim provides a number of online resources and has excellent suggestions that apply not only to Gaelic, but to any language spoken mostly by small, insular groups of people. Basically, you need to speak some every day, even when you only have a little bit of the language. The learner needs to shed their fear of making mistakes, and know that everyone knows mistakes are a part of learning. And if you can't find an immersive environment in person, you can get some of that effect via Gaelic radio on the Internet and (in some areas) Gaelic-language television.
It takes more persistence, and more effort to seek out the groups and networks of people who can help you practice. The learner also needs to understand a variety of cultural issues that can help or hinder the process. But with dedication and applied effort, it can be easier than you may think.
Gaelic speakers and learners are relatively lucky, compared to many of the endangered languages of the Americas: we have Internet radio stations, websites and good distance-learning programs, in addition to numerous courses in combined book and tape formats. While it's always easier to learn with others in a group, preferably with fluent, native speakers involved, it's possible to get a solid start on one's own.
Now back to increasing my skills anns a'Ghàidhlig ;-)