Your author hears countless complaints about the crowds at Notting Hill, but believes this is generally because people are spending all their time in the most crowded spots. One good way to enjoy the carnival on either day is to come from the North on foot or via Westbourne Grove Tube and to stick near the sound-systems, for example on All Saints Road, or by Norman Jay's Good Times Bus up somewhere near Southern Row (though your author understands that entry to Good Times may be only from Ladbroke Grove this year), as elsewhere it gets a bit busy.
The Southern Row end is especially good as most of the parades come along Kensal Road before dropping into the main parade route along Great Western Road, Chepstow Road, Westbourne Grove and Ladbroke Grove, and you can use Trellick Tower and the railway to stop you getting lost.
Anyway, your author will see largely none of it as he will be ensconced in a community centre nearby with the Paraiso School of Samba, ahead of the performance on Monday, which will hopefully be pleasant enough.
Sunday is kids day, though this only applies to the parades and the sound systems are the same, and Monday is the main day, then on Tuesday your author has to go back to work.
For more, the official website is http://www.nottinghillcarnival.biz/, or there's a decent guide in this week's Time Out.