While in the sketch right click and select fully define sketch to see if it helps.
Aside from the above idea of making your first entity close to scale, what I typically do is completely draw the sketch if I know what the profile will look like. Then add all of the dimensions - do not change any values, simply add dimensions to the sketch entities based on where they exist. THEN update dimensions to maintain functionality.
Until the sketch is fully defined there are a number of ways to satisfy the constraints which you may or may not anticipate.
also, several simple sketches solve faster than having one complex one - keep you sketches as simple as feasible.
it is also a good idea to follow this order:
2. add relations
2. add dimensions
Question. When you sketch a line do you have a box that pops and gives you the distance of the line you are creating. If not you should have this turned on when sketching. That way you sketch your line and see if you are close to your desired dimension. Say 2" vs 20"
Thanks all for the quick replies, really nice community! You all make good points and it looks like in the end it will come down to personal preference. The on screen numeric input looks particularly convenient, so thanks for pointing it out. Since I am new to this, it will probably take a little bit of playing around to figure out what works best for me.