Number of cases
When doing regression, the cases-to-Independent Variables (IVs) ratio should ideally be 20:1; that is 20 cases for every IV in the model. The lowest your ratio should be is 5:1 (i.e., 5 cases for every IV in the model).
Accuracy of data
If you have entered the data (rather than using an established dataset), it is a good idea to check the accuracy of the data entry. If you don't want to re-check each data point, you should at least check the minimum and maximum value for each variable to ensure that all values for each variable are "valid." For example, a variable that is measured using a 1 to 5 scale should not have a value of 8.
Outliers
You also need to check your data for outliers (i.e., an extreme value on a particular item) An outlier is often operationally defined as a value that is at least 3 standard deviations above or below the mean. If you feel that the cases that produced the outliers are not part of the same "population" as the other cases, then you might just want to delete those cases. Alternatively, you might want to count those extreme values as "missing," but retain the case for other variables. Alternatively, you could retain the outlier, but reduce how extreme it is. Specifically, you might want to recode the value so that it is the highest (or lowest) non-outlier value.
Multicollinearity and Singularity
Transformations
As mentioned in the section above, when one or more variables are not normally distributed, you might want to transform them. You could also use transformations to correct for heteroscedasiticy, nonlinearity, and outliers. Some people do not like to do transformations because it becomes harder to interpret the analysis. Thus, if your variables are measured in "meaningful" units, such as days, you might not want to use transformations. If, however, your data are just arbitrary values on a scale, then transformations don't really make it more difficult to interpret the results.
Since the goal of transformations is to normalize your data, you want to re- check for normality after you have performed your transformations. Deciding which transformation is best is often an exercise in trial-and-error where you use several transformations and see which one has the best results. "Best results" means the transformation whose distribution is most normal. The specific transformation used depends on the extent of the deviation from normality. If the distribution differs moderately from normality, a square root transformation is often the best. A log transformation is usually best if the data are more substantially non-normal. An inverse transformation should be tried for severely non-normal data. If nothing can be done to "normalize" the variable, then you might want to dichotomize the variable (as was explained in the linearity section). Direction of the deviation is also important. If the data is negatively skewed, you should "reflect" the data and then apply the transformation. To reflect a variable, create a new variable where the original value of the variable is subtracted from a constant. The constant is calculated by adding 1 to the largest value of the original variable.
If you have transformed your data, you need to keep that in mind when interpreting your findings. For example, imagine that your original variable was measured in days, but to make the data more normally distributed, you needed to do an inverse transformation. Now you need to keep in mind that the higher the value for this transformed variable, the lower the value the original variable, days. A similar thing will come up when you "reflect" a variable. A greater value for the original variable will translate into a smaller value for the reflected variable.
