For getting your spells to work, you have to consider 3 factors: yourself, the spell / ritual you are using, and the situation you are targeting. Personally, I think the term "magickal power" is a bit vague, as it could refer to any of these 3 factors I mentioned. So let's take a look at the angles to consider in order to get your spells to work:
1. Yourself - Many others have pointed to things like meditational, ritual, and other spiritual practices to help improve your "magickal" muscles. These are all valid and important for developing things like your psychic / astral senses, which are all important for magical acts. Additionally, keeping yourself purified and protected also falls into how to render yourself effective for doing rituals / spells. Another part that was mentioned was aligning yourself psychological with magick itself and with belief in yourself. Finally, another aspect for magical power in yourself is the energy work aspect. Being able to control and manipulate the energetic forces within you and around you can allow you to better affect the results of your spells.
2. The ritual / spell - You also need to consider the ritual / spell you are using. One fundamental element to this is if you are comfortable and willing to believe in the spell you are using. Does the belief system / origin of the spell correspond with spiritual beliefs you are comfortable in? Does the spell seem "powerful" enough to you to work? Answer these questions and discover what you feel most comfortable or happy with, because as another poster mentioned, you need to be psychologically aligned with the spell you are using for it to ultimately work. Beyond this, if you are using a spell that involves spirits, you may also consider using a proper offering approach the spirit. Buy a book like "Ritual Offerings" by Aaron Leitch and read up on how to give offerings to spirits involved in your spell / ritual. An offering can not only be seen as a way to pay the spirit for its work, but it is also a battery that helps to fuel the spirit's action in the physical world. This can be a powerful way to charge up your spells. But in the end, psychological alignment with the spell and its system is still more important than an offering by itself.
3. The situation - Ask yourself if it is possible to use a ritual to stop gravity from working or to do some other physically impossible act? I bet you will be disappointed if you thought it was possible to bend the laws of physics to your will through a spell / ritual. The final factor is huge in determining if your spell works or not. For a great breakdown of this, read "The Elements of Spellcrafting" by Jason Miller. In brief, a ritual or spell affects the probability of your desire or result manifesting into your life. For example, if your original chance to get a promotion was 10%, maybe a well executed ritual can increase that to 20%. But you always need to pay attention to the initial probability of your result happening with NO magical influence. For example, it is pretty close to absolute 0% chance of you being able to suspend the laws of gravity by magical spell. So then you ask yourself, what is 0 multiplied by anything? The answer is 0. If your desire is impossible or highly improbable to manifest by itself, a ritual / spell will do nothing to help you. However, if your result has a non-zero chance of happening, then a ritual can boost the odds or make something previously improbable more probable. If you add concrete action on your part to help the manifestation, then this boost's the ritual's effectiveness even more. So if your original chance of getting a promotion is 10% and the ritual boosts it to 20%, you could add another 10-20% on top of that by simply doing the things that would help you get a promotion normally. Your spells will work even better if you are the agent of your own change and you have a situation where your result can realistically manifest. It is all about smartly targeting the situation with your spell(s).
Altogether, these 3 factors working together can make your spells work better. The most important considerations of all 3 factors need to be used together to make your magick work. You need a ritual / spell that you believe in, a non-impossible situation to target the ritual's effects, and the commitment to seriously perform the ritual to the end. This is the minimum for what you need for a material situation to be successfully impacted by a ritual. The other stuff such as offerings, astral senses, and energy work can obviously help, but are not absolutely essential for this type of situation. But for a mystical or purely spiritual situation, astral senses and energy work become more essential. But in this situation, I am assuming you are more interested in the material side rather than the purely spiritual.
I hope this helps you.