If you do not plan ahead, you could unintentionally allow this year's Black Friday to give your church a black eye.  You love your church, and you believe in the work that the church is doing.  You sincerely want to see it thrive, and you believe God is blessing.  Even if you are someone who is not easily lured into impulsive bargain snatching, despite your best intentions, if you do not set aside funds now for your year-end donations, those donations will likely be weaker if they happen at all.
A little less than a third of all tax filers itemize deductions, but we give to God's work for reasons beyond the Internal Revenue Code.  For Southern Baptists, December is the month we emphasize international missions through the Lottie Moon Christmas Offering.  There are many opportunities for charitable giving, and I love what one speaker from the NC Baptist Foundation set forth awhile back.  Christians should give to Christian causes; who else is going to do so?  
Pray about an amount God would like for you to give, and go ahead and give those funds or set them aside if you can discipline yourself not to spend them later.  By taking care of what is most important first, you avoid the possibility of accidentally omitting your church from the cash outflows of the season.  Remember, whatever you do, don't give out of compulsion or obligation.  Give out of a cheerful heart, because you want to - not because you felt guilty after reading this blog post.