Reactions yielding alkenes often preferentially make certain isomers over others. Most often, this is because a particular isomer is more stable then another. Understanding what conditions lead to this greater stability helps in determining what products will be favored in a given reaction.

Cis or Trans?
Generally, trans isomers are favored over their cis counterparts
because the bulky groups are separated from each other and do not infringe upon the van der Waals radius of each other.

The cis isomer (left) has strain from the isopropyl groups infringing upon each other's radii. The trans isomer has significantly less and is therefore more stable.

Substituted or not?
Alkenes generally prefer to be connected to highly substituted carbons, a condition often referred to as Saytzeff's Rule. Most likely this occurs because of increased stability asa result of the added electron density from the alkyl groups to the pi bond and because of the additional separation between repulsing alkyl groups, going from sp3 hybridized to sp2 hybridized (from 109.5^o to 120^o) which reduces tension caused by steric effects.

The alkene on the left is more stable then the one on the right because it is substituted by an additional methyl group.