Here's a great article about how a good arrangement, at the beginning of your songwriting process, will help you get a good mix and loud master at the end: (link is only visible to registered users)
I agree with André. The problem is that due to the lack of specific mastering tools (like a look-ahead compressor and a proper limiter), it is hard to control the crest factor (difference between peaks and rms level) of your waveforms in Audiotool. As you increase the level, the peaks clip much before the RMS level has achieved a satisfying volume. But I also completely agree with him that loudness is more than just mastering. It actually starts with the arrangement, writing parts where powerful instruments like kicks and bass don't compete. Here's one of the best videos I've found that talks in depth about achieving loudness: (link is only visible to registered users)
instead of making the whole song louder try making sure each part of the song is at your prefered sound level