1. Your understanding of reactions - how each organic functional group tends to react with characteristic reagents
2. Knowing and be able to recognise the common mechanistic families of introductory organic chemistry
3. Know the variations within the families (e.g. nucleophilic subsitutions : SN1 or SN2)
4. Know the controlling characteristics of the mechanism subtypes (e.g. SN1 proceed via carbocations)

The most common reasons for students to struggle with these questions are:

• application of the rules of valence (e.g. the octet rule)
• poor appreciation of the "meaning" of line diagrams (e.g.thinking about where the electrons are)
• don't know the basic principles of drawing mechanisms (e.g. direction of curly arrows, making and breaking bonds)

Are there key skills good that help with drawing mechanisms ?  

• practice drawing good arrows, think about where they start and where they end (sloppy arrows can hide the meaning)
• practice so that "curly arrows talk to you" (they tell you what is happening)
• remember that curly arrows show electron movement not atom movement
• remember that curly arrows always go from electron rich to electron poor
• always balance the charges after each step
• get into the habit of showing lone pairs (esp. on N and O atoms)
• know how acid / base chemistry works
• use electronegativity differences to identify polar bonds and nucleophiles / electrophiles
• make sure that all bonding changes are accounted for with curly arrows
  

General strategy...

• identify the reactive functional groups in the starting material and product (if known)
• identify the type of reaction involved (e.g. it's a nucleophilic substitution)
• identify the subtype and then what controls that particular pathway
• think about the nucleophiles & electrophiles involved
• make sure to draw the curly arrows to account for all the bonding changes
• balance the charges after each step