As far as I can tell, Shounen fights are based around two things: movement and power.
Movement changes speed: it can go from slow and introspective to fast and intense (imagine a scene where a guy explains his awesome new technique and then actually uses the technique). Portraying movement and time through writing is a thing that every writer needs to learn even though it can be very difficult. I'd recommend reading old pulp stories (such as the works of Robert E. Howard) to learn some tricks you can use. They were always simple but action-heavy.
Power in Shounen is heavily based in Japanese spirituality. The physical power of a character is reflective on who they are as a person, what their virtues are and how strong are their beliefs. Goku from DBZ has the spirit bomb as his ultimate attack; it gathers the energy of living things to destroy evil. That demonstrates Goku's virtue of viewing life as precious and demonstrates his desire to defeat evil and protect those around him. That redhead dude from Yuyu Hakusho had a rose whip: whips are swift, elegant and fluid while roses are romantic. His weapon tells you about his character. Villains' power may reflect their cruelty, ambition and so forth. The stronger a character's conviction, the more powerful a fighter they are. Usually when a character is beaten in combat, it effects them emotionally and defeating the one who beat them is portrayed as a step in recovering from the emotional trauma of their defeat. It's all very Bushido and samurai-y. In Japanese culture, weapons and power are an extension of the warrior's will rather than tools that a warrior uses.
That's all I got. Hope it helps.
edited 28th Mar '13 11:32:59 PM by WSM