Don't wear loud accessories.
Those who engage in crossdressing at church may already be taking a big risk. The best way to go about it is to make sure the pastor and the congregation are OK with this practice and, if they are, to follow some common-sense dress rules. The proper dress code for crossdressers wearing dresses is not that much different than one for a modest woman. Except, of course, the crossdresser may have a much larger shoe size.
Going with a timeless, classic cut in a subtle colour is the best option for a proper crossdresser's dress. The style should be modest: Nothing low-cut that shows off cleavage, nothing sleeveless or backless, and nothing thigh-high or even above the knee. If you must pick a pattern, make sure it is subtle and attractive. Large plaid, loud flowers and garish colours are not proper church dress selections for any gender. Don't get too fancy. You are going to a church service, not a ball. And unless you are a bride, steer clear of long gowns, big trains and lace.
For proper fit, make sure you have the proper undergarments beneath your proper dress. Bras, girdles and seamless underwear are must for any formfitting dress to be flattering. Also make sure the bras have something to support and the dress top is properly filled out. Pantyhose or tights are a must for dresses that show any leg.
When it comes to the proper accessories to accompany your church dress, think low key. Stick with a plain, solid handbag that's not too large. You don't need one that jangles with trinkets, billows with feathers or looks large enough to serve as a suitcase. Your shoes should have low heels, a closed toe, and also be a solid, subtle hue. Keep jewellery subtle, perhaps studded earrings and a simple chain necklace. While hats used to be the norm in church, they are no longer necessary and may actually seem out of place.