Keep the brush light, don't push it into the canvas. That's the easiest way to get a smooth stroke, and it comes with practice. Also water is very helpful for blending, but be careful not to overload or your painting may appear washed out or "thin".
For shadowing/ blending colours, when you want to achieve a brighter more saturated effect, don't use BLACK to blend down in hue, instead use another colour such as a dark red, or a purple, or dark blue.. depending on what you're painting of course!
Also, develop your own method of layering/ steps to a finished painting. What I usually do is sketch on a pad of paper my ideas, and design from there before going to the canvas. Then I use pencil to lightly begin my design on the canvas. If you are doing a detailed background, start there. Begin by figuring out where your horizon line is if you are doing a landscape. Is there a sky? Do that first! Add other elements on top, this creates a clean separation between sky and earth. And always paint back to front. It is very annoying to paint AROUND an image that you began with. I'm sure all artists understand.
You can always use ink or felt pens to define your painting later to make it more crisp, if you don't want to use a very small brush or don't have one. Depending on your style, this brings more focus to the painting. I personally can't resist giving my paintings a final outline! And lastly, consider painting the edges of your canvas a single colour, like a black. This will frame your painting nicely.
Best of luck, fellow artists!