1. Think Proportions
Starting with your base layers build a triangle with your overall silhouette, either fitted on top or fitted on bottom. You don't want to overwhelm your figure with bulk on top and bottom. Although this can be a look into itself when layering it is best to avoid relaxed fit on top and bottom. Use fitted layers to create balance in the overall silhouette. For example a loose / boxy top or jacket over a slim pant or skinny jean or alternatively a full pant, or wide leg with a fitted top. This will create shape and overall balance. Then, add your layers over the top of this for example bulkier jackets work great with figure fitting top or slim pant underneath.
ALSO check out where the body is cut in half horizontally. That line needs to be tactfully chosen. Use belts to assist with this if you want to highlight shape. Cutting the body at a wide point emphasizes that width and usually we want to avoid this. Belt or "cut" at a narrower point. If you'd like to look taller dress in one colour, avoid the horizontal lines on your outfit, go for vertically striped fabrics or open coats/blazers to create one vertical line down the front of your body. If you're on the tall side and looking to reduce the illusion of height then wearing contrast top and bottom colours can help with this, or layering to create horizontal lines, use of a belt or even a shirt hem coming out from the bottom of a sweater.
2. Use Colour
There are many ways to use colour (this is a big subject which we can't get into in one post!) In the case of layering and to make things simple we suggest one of two routes. Either go with all one shade (or similar shades say blacks and greys, or tans and browns) and add interest with accessories OR contrast throughout the outfit by choosing two colours and matching top and shoes with a contrast pant. Example black tee with white jeans and black loafer. A really easy option is all one colour with base layers (say black or navy top and pant) and then a contrast coat or blazer layer (see image below). Have a play and see what suits you best. This is a creative activity but once you have some formulas in your mind it's simple.
3. Consider Textures
Use texture to create interest. Consider how fur is used on coats, adding garments of a similar colour but of varying textures is a great layering formula. Here is an example: Leather jacket, dark denim and a wool base layer such as a merino -- so easy, all black but the various textures make it far from boring. This is an excellent way to fool proof your layering and outfit combinations in general.
Use accessories to add personality to your overall look and help with the proportions and balance throughout the day. Long necklaces or scarves that follow the neckline of a jacket or blazer are like a "layer" unto themselves and they maintain the outfit balance if the jacket or blazer layer is removed in the day. Use these sparingly but wisely!
5. Know Yourself
At the end of the day it's all about confidence so if you're not feeling it, then ditch it. It's got to be in alignment with you and if you're self conscious or uncomfortable that's going to shine through. So, at the risk of sounding very cheesy, be true to yourself.