Ctrl-I F will display all files except the currently selected filespec. And - an undocumented command - Ctrl-F will select the last filespec used.
In conjunction with S (Showall) or G (Global), Filespec is especially useful for finding an elusive file: in the S or G window, the path of a highlighted file is shown on the top line.
XTree maintains a History of previous filespec selections, accessed by the up-arrow. I have two retained lines in the History:
*.COM *.EXE *.BAT to show all executable files and
*.DOC *.TXT READ*.* *.WRI *.ME to show all documentation files.
Tom Ruben