I first thought this was a no brainer, because it almost always means it's going to get easier.
eg: You've biked the hardest uphill part of the race and it's all downhill.
It's all smooth sailing from now on.
But actually now that I think about it, although it usually means something positive, it depends on the context. If you are talking about a business's future, it could be interpreted as meaning that the success of the business is crumpling; going down. But in the same sense it could also mean it's easy for the business from now on.
It's all about the context. However it could be misinterpreted by those unfamiliar with the phrase, so if you are trying to be tactful, avoid it.