Over at The Hardball Times, Paul Singman has a column on the factors to take into account when spot starting a pitcher. 

From research and experience, we have some additional things to investigate when rolling the dice on a starting pitcher. These data points have helped us settle whether to gamble on a pitcher who hasn’t shown the consistency yet to be counted upon for good performances in at least 60 percent of starts.

• Last three starts: Pitchers tend to be streaky creatures of habit. Finding a good arm slot is often enough to get onto a roll. Additionally, don’t underestimate the influence of confidence in a pitcher’s performance. A game where a pitcher allows a devastating amount of runs has a psychological impact that can take weeks to recover.
• Strikeouts, strikeouts, strikeouts: We look at two things. A pitcher’s strikeout rate and the opposing hitting club’s strikeout numbers. Simply put, a pitcher who racks up a lot of strikeouts has a better chance of performing well. But also, a lot of strikeouts raises the upside of a great start and caps the downside of a bad start. Even if the pitcher kills your fantasy team in ERA, you’d like him to be able to contribute in at least one category.
• Historical record versus team: Low sample sizes can often be deceiving, so there’s not much to read from the last time your pitcher faced a team. But noting how a team’s best batters have fared against your pitcher over several matchups can help to tell how well these players see the ball come out of your pitcher’s hand. Also, some pitchers tip pitches and some teams have very good scouts services. Information is passed around. Track record definitely can be a factor in predicting a good or bad matchup.
• Opposing ballclub’s OPS against pitcher’s handedness: Some teams are loaded with left-handed batters and hit right-handed pitchers well but left-handed pitchers poorly. Others are the reverse. For instance, at the moment, the Oakland Athletics, Pittsburgh Pirates, and Baltimore Orioles have 65 points of lower OPS against left-handed pitching. The Houston Astros, New York Mets, Detroit Tigers, Cleveland Indians, and New York Yankees have 65 points of lower OPS against right-handed pitching.
• Ballpark factors: Everyone knows that Petco Park in San Diego is a great pitcher’s environment while Ameriquest Field in Texas is a great hitter’s environment. It also helps to match up your pitcher’s skills to the ballpark’s propensities. Obviously, you don’t want to spot start a fly ball pitcher in Colorado, but just as damaging could be the starter who pitches to contact and allows a lot of balls in play in San Diego. A spacious ballpark that doesn’t allow a lot of home runs can still be dangerous. Furthermore, some ballparks favor right-handed batters and some favor left-handed batters and so it’s worth matching your pitcher’s handedness tendencies with the ballpark’s handedness tendencies.