ANSWER - I'm not so sure that what you are describing is a limitation. I usually find that people with a mind that tends to wander have a higher level of imagination than others and imagination and creativity go hand in hand as being very key ingredients in effective spell work. Don't get me wrong, maintaining a certain level of focus is important but I think there is also a lot to be said for letting the mind to sit back, reflect and allow random ideas and concepts to float in at their will.
The easiest way to maintain concentration is to keep your mind interested in the task at hand and you can tailor your spells to make this your preferred way of working. The first thing I would suggest is that you use a very tight structure for your workings. If you know your mind is prone to wandering write your spells out beforehand so you are not relying on your memory. Same with every step of the working itself - create yourself a step by step guide to each stage of what you want to do so you can follow it as you go. Holding a single thought in your mind can be tricky for even the most accomplished of witches but having a definite idea written down on a piece of paper will help your keep your mind on the task at hand.
Now this idea might sound a bit magically unconventional but I think you could enhance your concentration still further by incorporating a puzzle element of into your spell work. If you can keep the forefront of your mind stimulated into solving a puzzle then it will allow your subconscious mind to relax and focus on the intent of the spell. Strange as it may sound I'm going to suggest you buy a simple jigsaw (nothing too complex you don't want to be at it all night - say 50-100 pieces) and you complete the jigsaw while focusing on the spell. You could easily visualise this as part of your spell work - particularly for a luck spell. As you fit the pieces together you can visualise all the things that you want to work for you coming together and joining together to form the new situation your working is intended to achieve. The combination of the mental power involved in completing the jigsaw and the visualisation of each jigsaw piece as a symbolic part of the overall working should allow you to focus efficiently on the outcome you want your working to achieve.
Give it a go and let me know how it works. I think for you it's just a case of distracting that very overactive conscious mind of yours!