First, there is certainly no universal answer to this. There are many factors, and a fair amount of them have little to do with your work itself, and more to do with your goals, tolerance to risk, etc.
I have fear that high hourly rate could reduce work/project consistency.
I think in your case, that's your answer. Since you're billing hourly (which is what I do), you can look at it as supply / demand.
When should you increase your price? When there is more demand than supply. Which is to say, when you turn down or delay new work because your schedule is full.
At that point your rate becomes a way to reduce demand until it matches the available supply. So if you're currently working at capacity, this method would have you increasing your rate incrementally until you reach a point of equilibrium.
Of course, freelancing is a bit volatile by nature, so 'equilibrium' won't really mean you never have to delay a new project or have some downtime between projects.
But supply / demand isn't a perfect model (at all) when it comes to your time. As Peter points out you could benefit from increasing your rate to a point that you have more time (supply) than work (demand) yet be making the same or more. I touch on this briefly during the 'I Don't Scale' section of my 'Adventures' talk.
Another factor to consider when it comes to hourly rates is that some clients won't be interested in you because of your rate. But that cuts both ways. It's not just clients that can't (or won't) afford higher rates, there are also clients that won't be interested in low rates because of what a low rate communicates to them.
You can raise rates without surplus demand as long as the value of your time has increased. Or, to state it another way, as long as you understand clients will have a different expectation when paying $50/hr than they do when paying $10/hr. (At least good, competent clients will.)
I would like to know what is base which help to decide hourly rate up down.
Not the only list that could be put together, but a tl;dr; of the above:
- Are you working at capacity? Increase incrementally until demand matches supply.
- Are you tired of working at capacity? Experiment with increases that keep your income the same, but reduce your time.
- Has your skill or ability increased significantly? Increase to reflect the value, understanding that client expectations will change as well.