The lip buzz is a good one. Relax, warm up. Do a few yawns, some humming, a few falsetto descending siren sounds. Stretch. Transpose new stuff down an octave. Spending time with a voice teacher is a good idea. Also, singing in a choral group helped me a great deal. I'm a baritone, but with some training and discretion I have been able to augment some rather thin tenor sections over the years (truth is, there are a lot of guys who think they are tenors, but there aren't that many of them.) Tenor choral parts quite frequently don't go above G above middle C. Being able to fake one part and really sing another enhances your ear for harmony. Remember, electric 4/4 music is a microphone game. MIchael Stipe is a baritone, but he showed he could exploit the mic with some falsetto action in "Kohoutek". Don't strain those falsetto attempts - keep it light and pretty and let the mic do the work, and arrange the song so your falsetto highlight isn't drowned by everybody playing loud. Chances are there are a few falsetto notes above what you can sing with power...with a little practice, a baritone D, E, or F above middle C sung in full voice can sound splendid. I am a fairly new guitarist (was a drummer before I got into singing) but it seems to me that C, D, or E are good, playable keys for baris. Singing books that are written with the typical voices in mind (baritone for men, "altos" or "mezzosopranos" for women") tend to feature E above middle C as the highest note sung.
Remember, music isn't math, and higher isn't necessarily better. A baritone with a little charisma and imagination (think Sinatra) does just fine.
