You can be a real jerk and make a word that has no vowels. Here are some examples.

TSKS
TSKTSK
TSKTSKS
PSST

If your friend gets mad at you, tell him this was Peer's idea. The following words contain the vowel "Y," which might as well not be a vowel at all!

BYRL
BYRLS
CRYPT
CRYPTS
CYST
CYSTS
DRYLY
DRYS
FLYBY
FLYBYS
FLYSCH
GHYLL
GHYLLS
GLYCYL
GLYCYLS
GLYPH
GLYPHS
GYMS
GYPS
GYPSY
HYMN
HYMNS
HYPS
LYMPH
LYMPHS
LYNCH
LYNX
MYRRH
MYRRHS
MYTH
MYTHS
MYTHY
NYMPH
NYMPHS
PSYCH
PSYCHS
PYGMY
RHYTHM
RHYTHMS
RYND
RYNDS
SCRY
SHYLY
SLYLY
SPRY
SPRYLY
STYMY
SYLPH
SYLPHS
SYLPHY
SYNC
SYNCH
SYNCHS
SYNCS
SYNTH
SYNTHS
SYPH
SYPHS
SYZYGY
THYMY
TRYST
TRYSTS
TYPP
TYPPS
TYPY
WHYS
WRYLY
WYCH
WYND
WYNDS
WYNN
WYNNS
WYNS
XYLYL
XYLYLS
XYST
XYSTS
@@Words that End with E
One thing I've noticed is that if the revealed letter given to you is an "E" in the second to last position, the last letter is most likely an "R", a "D", or an "S". Your opponent may be making a word like "trad-ER", "trad-ED", or "trad-ES". Conversely, when it's your turn to make a word, you may be able to stump your opponent for a bit by placing an "E" in the second to last position and making a word that ends in "-EM" or "-EN".