when we rub objects together, what determines which material will pick up electrons? For example We know glass when rubbed by silk will become positively charged while the silk will be charged negative. 
What exactly makes glass appropriate for losing electrons in that experiment? (
 A: There is no single factor that uniquely determines whether a certain material is going to gain or lose electrons, but one of the most important properties is the electron's affinity of a certain material. If the affinity of a material is negative, that material will most probably be a receiver of electrons. If the affinity of a material is positive, that material will most probably be a donor of electrons. You can find here a detailed timetable showing  the relation between the tendency of receiving / donating electrons and the electrons affinities for a list of materials. 
That is not a super-strict role, it is more of a strong correlation. The charge transfer between two bodies depend also on adhesion mechanisms and the electrochemical potential, you can find some details here
A: Each object has a certain value that defines whether it will lose electrons when rubbing against an object with a higher value, and vice-versa. Glass can loose electrons much more easily than other objects, making it a useful tool for teaching about static electricity. Electrons are held in place by the positive balance of protons.
"different materials have different affinities for electrons. By rubbing a variety of materials against each other and testing their resulting interaction with objects of known charge, the tested materials can be ordered according to their affinity for electrons. Such an ordering of substances is known as a triboelectric series. One such ordering for several materials is shown in the table at the right. Materials shown highest on the table tend to have a greater affinity for electrons than those below it. Subsequently, when any two materials in the table are rubbed together, the one that is higher can be expected to pull electrons from the material that is lower. As such, the materials highest on the table will have the greatest tendency to acquire the negative charge. Those below it would become positively charged."
Links:
http://www.physicsclassroom.com/class/estatics/u8l2a.cfm
A: The process of transferring charge between two objects by bringing them into contact and/or rubbing them together is known as the triboelectric effect, and is historically, the first recorded observation of an electric phenomenon.
Wiki Page on Triboelectric effect
In general, there are too many competing factors such that there is little predictive power.
A: electron-affinity of the material. Condition of the surfaces of the materials. The intensity of the rubbing procees-hard or gentle
A: It depends upon the triboelectric series which is:
$$\text{fur > flannel > wool > glass > paper > cotton > silk > wood}$$
The material which appears earlier in the above series has a higher tendency to get charged positively.
