One of the few universal truths is that human body functions are funny. We don't know why, but then we don't give a flying fart as to why (note use of body function for humor). Comic Art is an applied science, not a theoretical one.
1. Sneezing is funnier than coughing because it has flying snot (see rule 3 below). A suppressed sneeze is funny as well, because then the snot can shoot out the ears. Smoke coming out of ears is funny too.
2. Burping is funnier than sneezing. Farting is funnier than burping.
3. As a general rule, the lighter the body fluid color, the funnier it is. Phlegm, saliva, sweat, and tears are funnier than urine and ear wax. All are funnier than blood, which is funnier than s***. The exception to this rule is boogers, which are green but become dirty after being rolled around between the thumb and fingers.
(Bonus rule: always disguise naughty words with *s; everyone knows what they are anyway. This is the text equivalent of the black-bar-covering-naughty-bits rule, which was discussed in the basic course.)
4. Showing urination and defecation is not as funny as some might suppose. It's rather juvenile, but if your audience is, too, then use these functions, but be sure not to show them graphically. Peeing should be done with the back turned, a stream flowing from the hidden source, or as a puddle at the feet after someone has pissed his or her pants. S***ing is best when implied, as in a pair of legs with dropped trousers seen beneath a toilet partition. A fly hovering about is a nice added touch.
5. Body odors are funny, in ascending order: arm pits, feet, breath, farts. A few wavy lines (phew marks) emanating from the offending area will convey this concept. Once again, a fly hovering about is a nice added touch. Better still is something wilting from the stench. The less likely a thing is to wilt the better; a wilting flower is less funny than, say, a wilting sword or cannon barrel.
6. Puking seems to be all the rage in movies and on TV for both dramatic and comic effect. Graphic puking is for hard-edged realism; humorous puking is usually done off stage. There is a lesson here for the comic artist.
7. Lactation and ejaculation are not funny. Motherhood and fatherhood are funny but not on a functional, biological level.