Concentration is naturally increased by doing more and more bhagtee and this is the best way concentration will increase. Guru Sahib states...ਸੁਣਿਐ ਲਾਗੈ ਸਹਜਿ ਧਿਆਨ, by listening over and over eventually our mind will connected with Gurbani.
While doing Path we should activlely fight against our mind and not just do Bani is a lazy state. If we are tired then we must fight against it, and never let the mind waver. The least amount of thoughts are during Amritvela and if one is tired during AV, there mind will never be able to focus on Gurbani. It is important to get a good amount of rest before Amritvela.
If one wants to speed up the process, than it is crucial to learn Gurbani Viyakaran and meanings. Even with knowing little punjabi one can come to understand Gurbani if they put their mind to it. This way, when doing Path focus should be put on the meanings and by knowing studying Grammar it will keep you constantly active to look for where bindian, audaks, bishrams etc. should be put. In order to do this one has to do path with a good amount of concentration. At the begining it will takes some time to understand the meanings at a fast pacebut with practice eventually it feels like one is speaking a normal language and you understand at no matter what speed you go at. Gurbani should be understood just as we understand speach in everyday life (meaning you shouldn't have to stop and think about it). For instance, you are not thinking about what i am writing right now, but you are just reading it at is processing in your mind right away. Knowings the meanings also helps with interest in path and leads leads to vismaad. Speed should be increased once Bani is completly shud or else this can lead to many errors.
Rehit is also important, those that keep full Sarbloh Pehra, Bana and other Rehits will all tell you how much their concentration increased after keeping rehit. The more we follow Rehit the more Kirpa Guru Sahib will do. This Kirpa can be translated into many things such as warding off illness, dukh, and worldly affairs but most importantly Guru Sahib increases ones Bhagtee and Concentration. If one eats from a manmukh, than that persons actions goes right into the mind. This will lead to increase in vikaars and inability to focus on Gurbani. Guru Sahibs Bana is a magical thing and makes one feel like a Bhagat. It isnt just for fun that Guru Sahib would make his Singhs wear Bana. Bana is not something that is for physical battles but also spiritual battles and it gives one the feeling of being Tyar bar Tyar, which helps fight off negative thoughts while engaged in Bhagtee.
When one is doing Bhagtee while sitting, it is best to choose a comfortable place, with little noise and distractions. It is also good to sit in the same spot while doing bhagtee because the spot becomes more and more pavitar (purified) as we do more bhagtee in that one area. We also become familiar with the surroundings. The Singhs here in Toronto have become so accustomed to their "Bhagtee spaces" that we will never sit in any other spot during our morning Naam Abiyaas. A habit has been formed between us so strongly, that even when one person is sitting out of place, the mind begins to waver. I remember a few occurrences where the person who normally sits beside was gone and someone new had accidently sat in his space. I spent the whole abiyaas thinking about 1) Who this new person was 2) How annoyed must the Singh who orginally sat there must feel! One should always bundle up as well, and wearing a loee really helps. It keeps one warm and gives the feeling of being a fakeer. It is also helps to use a mala while doing Naam Abiyaas.
Aside from the Major rehits, we should look out for the minor things that also effect our jeevans. Watching Movies, playing video games, and listening to dirty music all have negative impacts on the mind. Even if they are not bad in nature, the thoughts of these will continually pop up, especially when we try to do bhagtee because our weak minds have come to enjoy them so much that we cannot stop thinking about them.