In terms of transmitting or contracting HIV during sex between women, safer sex is any type of sex which does not allow one woman's blood, vaginal fluid, or breast milk to get inside another woman's body. The following information is a list of some basic guidelines about how to prevent HIV infection between women, and they apply to the prevention of transmitting many other STDs (that are transmissable through vaginal fluids) as well, like Chlamydia, Gonorrhea, Hepatitus B, and Trichomoniasis. There are some STDs, however, that can be transmitted via skin-to-skin contact, like herpes, HPV and crabs, for which the precautions are different.

Deep Kissing
Deep, wet kissing is considered a safer activity UNLESS there is any blood in either of your saliva, or if either of you has any open cuts or sores in your mouth, or bleeding gums. Wait at least 2 hours before and after brushing your teeth to engage in deep kissing or any other type of sex. Saliva doesn't contain a significant enough amount of the virus to transmit it, however, it can be risky if there is any blood in either of your saliva.

Manual Sex: Fingering & Fisting
Manual sex is the use of your finger(s), hand(s) or fist during sexual activity with your partner(s). To be safe during manual sex, wear latex or nitrile gloves. A good trick is to put two gloves on initially, and when move from your partner's anus to her vagina, or from her vagina to yours, etc., simply take off the top glove and continue with the clean one. If you use a lubricant, make sure its water-based because oil-based lubes can degrade and damage latex.

Contact with menstrual blood
Coming into contact with menstrual blood during oral sex can be very risky. If your partner is infected with HIV, her menstrual blood (like other blood) will have a significant amount of virus in it, and getting blood inside of your mouth can definitely transmit the virus. Having any cuts or sores on or inside your mouth can also increase your susceptibility of contracting HIV through oral sex. HIV can also be transmitted through tiny cuts or sores on your hands. If you're having manual sex (fingering & fisting) with your partner(s) while she has her period, use a latex or nitrile glove as a barrier.

Oral sex
Performing oral sex on a woman can be risky, (especially when she has her period!) because HIV thrives in vaginal fluids and blood (including menstrual blood). To make oral sex safer, keep your partner's fluids out of your mouth. Cover her entire vulva with a latex barrier (dental dams, or a condom cut up the middle and spread flat) or a piece of non-microwaveable plastic wrap. If you're putting any part of your mouth and/or tongue on or inside her anus, be sure to use one of the barrier methods mentioned above - there are a host of serious infections that can be spread by ingesting someone else's fecal matter, like Salmonella, Hepatitis A, Schigella, or intestinal parasites. Also, if you move from going down on her anus back to her vagina, use a new barrier.

Sex toys
Sex toys are safe unless you're sharing them with your partner(s). If you're sharing sex toys with other women during sex, be sure to wash the toy thoroughly with warm soapy water (or put a condom on the toy and change the condom) every time you switch from one person to another, or from a woman's anus to her vagina. Use plenty of water-based lube to reduce vaginal or anal tearing or trauma during sex with toys. Reducing the trauma and tearing that can take place during sex through entry, friction, and force, for example, can also reduce your risk for transmitting and contracting HIV and some other STDs.

S&M activities
S&M play is safe unless there's any blood involved. If your activities involve cutting or piercing yourselves or one another, be sure to clean the needle or razor with bleach and rinse thoroughly between users. Use different razors if you shave each other. Never allow the urine or feces of another person to get inside your mouth, eyes, nose, vagina or anus.

For more information on S&M Safety check out this previous post:

S&M Safer Sex Article

Other safer sex includes mutual masturbation (if you keep your hands to yourself or above the belt), and nipple and breast play (as long as there's no breastmilk involved).