VBA Help required, issues with sorting.

Hey guys, i'm having a problem here, i'm trying to create a macro to sort a range of data, I can get it to select the whole table easily, but the problem is after I create the macro, if I wanted to add further data to it, it misses out that data when doing the sorting process.. If anyone has any recommendations i'd appreciate it, here's the code...

Code:

Range("A1").Select
    Range(Selection, Selection.End(xlDown)).Select
    Range(Selection, Selection.End(xlToRight)).Select
    ActiveWorkbook.Worksheets("Sheet1").Sort.SortFields.Clear
    ActiveWorkbook.Worksheets("Sheet1").Sort.SortFields.Add Key:=Range("M2:M18") _
        , SortOn:=xlSortOnValues, Order:=xlDescending, DataOption:=xlSortNormal
    With ActiveWorkbook.Worksheets("Sheet1").Sort
        .SetRange Range("A1:M18")
        .Header = xlYes
        .MatchCase = False
        .Orientation = xlTopToBottom
        .SortMethod = xlPinYin
        .Apply
    End With