In reply to Dan Lane:
We are out to harris and Lewis at least once a year and normally only book a few days before(travelling on Saturdays). The only time we've had any problem was when we trying to travel the weekend the Mod finished. The Hebfest weekend in July might be a problem also. Thats travelling with a camper as well, foot passenger only is normally easier. In otherwords, book as early as you can, but don't be too stressed about booking weeks in advance.
Not sure about the Uist routes though, only times we've been down there was with bikes so just turned up and got places on the ferry normally.
Barra is well worth a visit - a great fun place, and the pub in Castlebay is good and lively.
Try the Indian there if it's still open, it was excellent when we were there last - and scallop pakora needs to be tried!