Buffering preloads your videos in order to avoid fluctuations in your Internet connection from causing choppy playback. However, while buffering is actually meant to help with smooth playback, sometimes it's the problem preventing smooth viewing. The issue likely lies with your Internet connectivity or browsing habits.
Your Internet Speed
YouTube, Vimeo and DailyMotion help articles all concur that your Internet connection's speed is the most likely culprit behind slow or choppy playback brought on by buffering issues. YouTube suggests using at least a 500-Kbps broadband connection, and you can check with your Internet provider or consult your bill if you're unsure what speeds you're paying for. In addition, run an online speed test to see if you're getting the bandwidth you're paying for (see Resources). Note that if this is the issue that's causing slow buffering speeds, the only sure-fire solution is to purchase a plan with higher Internet connection speeds.
If you notice video buffering takes longer during specific hours, network or site congestion could be the issue. If the site is experiencing higher traffic than normal, it can cause dips in performance. Similarly, when other customers using your Internet service provider are the most active, it can negatively impact speed. Think of it like a traffic jam -- with so many people getting off work at the same time, the freeways get congested and things move much more slowly than they did an hour earlier. Same goes for the Internet: when a lot of people in your area are using the same ISP you are, it can cause an overall performance slowdown. Find the best time to watch your videos by trying to watch them at different times to see if it makes any difference.
Other Bandwidth Hogs
For optimal playback, close other browser windows and tabs. Close any programs that use an Internet connection, as well. Since your Internet connection isn't an infinite well of bandwidth, other programs, windows and tabs can negatively impact video playback. Think of your bandwidth like a pie and your current connections as people; there are only so many slices, and the more you try to make, the less pie each person gets. Closing the other bandwidth hogs enables your browser to give your video the bandwidth it needs to buffer quickly, without other connections dipping into that speed.
If the site you're using allows users to adjust video quality, choose a lower quality. A video with 240-pixel resolution loads faster than a 1080p HD video since there's less data to deal with. YouTube's help article on the subject suggests clearing your Web browser's cookies and cache, and then trying again. A large cache that isn't regularly cleaned out can negatively impact browsing speed. The simplest solution to buffering issues is to open the video, pause it, and then do something else while it buffers. Sites typically have a progress bar that shows how much of the video is buffered, so wait until the bar is full or close to full before you play the video.