You may want to check out:

http://www.cityofboston.gov/parks/hbgi/iconography.asp
http://www.pacificgrids.com/skull/winged-skull-article.htm

Please bear in mind there may be local bylaws prohibiting gravestone rubbing, please always check first to avoid falling foul of the law.

Monuments are fragile:
Do not sit or lean on monuments. Often monuments are not securely attached to their bases and could topple over causing injury or damaging the monument. Believe it or not, people have been killed when monuments have toppled over.
DON’T EVER, EVER, DO TOMBSTONE RUBBINGS!! The practice of taking rubbings from monuments has been made illegal in many localities – and for good reason. The physical act of attaching the paper can leave tape residue and the rubbing of pencil or charcoal over the monument will, over time, wear down the edges of inscriptions. Also, stone often deteriorates from the inside out as moisture is drawn up from the ground by the porous stone. There may well be on empty cavity inside a monument which looks stable. This space will collapse if you try to rub over it. Taking rubbings of a monument is a selfish, destructive activity: in a word, it’s vandalism.
Don’t spray or rub anything into monuments in an attempt to make the inscriptions more visible. Believe it or not, I have actually heard of people who rub shaving cream or chalk into tombstones so that the inscriptions stand out. Again, this is vandalism. There are many non-invasive ways of reading an inscription. Often a simple white card to reflect the light at an angle, or waiting for the right time of day, will make the inscription jump out. It is better to wait a couple of hours rather than risk damaging an historic monument.

Otherwise enjoy your hobby. Regards, Nick