Loki Astari's ideas will work (for the most part) except for the last one, in one special (but certainly no less important) case: If the string being split is all spaces.
This can be fixed with a simple conditional statement:
std::string trim(std::string const& str)
{
std::size_t first = str.find_first_not_of(' ');
// If there is no non-whitespace character, both first and last will be -1
// There is no point in checking both, since if either doesn't work, the
// other won't work, either.
if(first == -1)
return "";
std::size_t last = str.find_last_not_of(' ');
return str.substr(first, last-first+1);
}