I do NOT think that it is a mortal sin to think about a venial sin, recognize that it's a sin, and then do it. A venial sin, is a venial sin, is a venial sin. No amount of venial sins stacked together will ever equal a mortal sin. A venial sin opens the soul to mortal sin, will lead one in, and makes it much easier to do. Example: It may only be a venial sin to eye the figure of a passing girl, with no lust in the heart. However, this lack of custody of the eyes leads one directly into the mortal sin of lust, and it's a slippery slope.
A sin of any kind, as I understand it, requires knowledge that it's a sin, the WILL to do it, and the fact that the action in question is a sin. Therefore, a sin of, say, "the little white lie," would be a venial sin, and included in that would be that it is known and wilfully done. If one had a lifestyle of little white lies, that would be very bad, simply because one takes one's salvation into one's own hands by opening oneself to these huge temptations... playing on the top of the slippery slope, so to speak. One would be greatly tempted and much more likely to fall into the mortal sin of larger lies. However, in and of itself, the commitment to commit this venial sin every week, or every day, or three times a day, will never of itself be a "mortal sin."