I'm just (I mean JUST) starting to make it as a jazz singer, so I don't know if I'm the right one to give advice, but here goes:

(1) LISTEN to the great vocalists.  Even though you're a male, most of the great jazz vocalists of the past were female, and you can learn a lot.  I think that's partly because that was the only role in music allowed for females in those days.  You will see huge bands and small bands (black or white) with all men except the vocalist.  So the only thing a good musician of the female type COULD do in those days was be a vocalist...

(2) Learn music theory!  There is no substitute in jazz.  You have to learn to hear the chord changes, because you can never be sure who will be played what behind you--but someone somewhere is likely to be playing the root of the chord.  Learn to sing the melody by practicing with ONLY of the root of the chord (from the chart, that is "fake sheet"), or with the root and the 7th.  Be aware of what note in the chord the melody line is, or whether the melody line is "passing tones", chromatic, or other dissonances.  Remember, the band will NOT be playing the melody.

(3)  Go to jam sessions--observe the protocols.  Some jam sessions don't welcome vocalists.  If they don't, find another, because you can bet if they don't welcome vocalists they won't play well or sympathetically with you.

(4) Be prepared.  Know what song or songs you are going to call. Remember, in a jam session they may have already just done what you planned, so have a couple of backups.  Make up your charts in the key YOU want to sing.  Don't depend on the band being able to transpose in all 12 keys.  They're supposed to be able to, of course, but...

If you have time (or a computer), make up a chart for B-flat and E-flat instruments also, not just the C chart. 

Make your charts clear and legible (remember, it might be dark in the bar or restaurant).  Use 4 bars per line, make a note where the verse, chorus, and bridge are, and if there is any tag or special ending where that starts.  If there is a "usual" jazz intro for a song (e.g., "All the Things You Are", make sure you know it, because chances are the band will play it.)   If you want a different intro, say so, e.g., "give me a four-bar vamp on B-flat." 

All this should be obvious to good jazz musicians without your notes, but it never hurts, and the players will respect you if you have a good chart and give good instructions.  

However, be careful how many instructions you give in a jam session.  Keep it pretty straight.  When you have your own band in rehearsal, that is the time for working out your own individual arrangements.  In a jam session, the players come to PLAY!

(5) Be able to count off the time.  Count 1-2, 1-2-3-4.  Make sure that you know how to count off to get the tempo you want.  Get the band's attention, and be firm and accurate.  If you are singing part of the song "rubato", make sure your signals are clear.  It's also helpful to tell the band if you are going to tag the song, and if so, how many times.

(6) Remember that for better or worse, the vocalist is the leader.  That means you must signal to the band who and when to take solos.  You have to let the rhythm section know when you are taking a break.  You have to let the band know whether you are coming back in at the head or the "B" section.  And you have to PAY ATTENTION while the players are taking their solos!! That is not the moment to let your mind wander until it's your turn again--visualize that you are the conductor.

If you get lost, listen to the DRUMMER.  You can tell where the head section or B section ends, because the drummer will change his/her pattern noticeably.  Also, know ALWAYs where the 1 (one) of the bar is.  The drummer will play the 2 and 4 with the high-hat (usually), so that's another clue. 

(7) Learn scat singing even if you don't intend to use it (I don't use it myself, but it's helped me TREMENDOUSLY in understanding the music).

(8) Although the usual karaoke tapes are OK, it's better to get the Jimmy Abersold tapes/cd's.  Get an issue of DOWNBEAT magazine, or if need be, I can give you the info to order.   For a while, stick to what's in the jazz fake books or on those tapes.  I once went in to a jam session with "How Are Things in Glocca Morra," a beautiful song, and not particularly difficult.  But the piano player had never heard it, and even though he was a good sight-reader, it was a DISASTER!