After suffering a 5-0 Ashes drubbing at the hands of the Aussies, England then stunned the world champions by winning the Commonwealth Bank one-day series.
That has led to increased expectations that England can finally win a World Cup title after 32 years of heartache.
"When you land in the West Indies, there's a different atmosphere so we've just got to make sure they don't switch off," he told Sky Sports News.
"The other thing is we have to make sure they don't get a little complacent after that good result.
"I don't think they will because they are a bunch of individuals who want to succeed, and they've got the character as they showed in the comeback in that series.
"We're pretty confident, there's a buzz about the side so let's maintain that. At the same time, we must not get over-confident."
England face Bermuda in the first of two warm-ups here on Monday and then face Australia again on Friday.
Their first round campaign gets underway on March 16 at St Lucia against New Zealand followed by further Group C games against Canada and Kenya.
Fletcher still believes Australia, aiming for a third successive World Cup title, are the team to beat.
"From results over the last month or so it's going to be pretty open," said the coach.
"I still believe Australia are favourites, but it's opened up a lot and there are a few sides who have come to form at an important stage."