If your Web browser can't display the "™" correctly - it is supposed to show up as a [TM] marker - get yourself a decent browser that does support the standard. Other nice signs & symbols are:
<!ENTITY emsp   SDATA "[emsp ]" -- em space [X X] -->
<!ENTITY ensp   SDATA "[ensp ]" -- en space (1/2-em) [X X] -->
<!ENTITY mdash  SDATA "[ndash]" -- em dash [X—X] -->
<!ENTITY ndash  SDATA "[ndash]" -- en dash (1/2-em) [X–X] -->
<!ENTITY nbsp   SDATA "[nbsp ]" -- non breaking space [X X] -->
<!ENTITY shy    SDATA "[shy  ]" -- soft hyphen [X­X] -->
<!ENTITY copy   SDATA "[copy ]" -- copyright sign [X©X] -->
<!ENTITY trade  SDATA "[trade]" -- trade mark sign [X™X] -->
<!ENTITY reg    SDATA "[reg  ]" -- registered sign [X®X] -->
(Oh, fuck! I just tested them on the very holier-than-thou Arena 0.95b itself [in case you don't know - this is the HTML-3 test-bed browser supposed to implement the very latest standard], and even she doesn't get them all right. There may be a font problem, but a broswer should be able to substitute combinations or alternatives for symbols of known semantics. It is not that hard to implement a rational approximation to differently-sized spaces and dashes. This is no rocket-science!)