Tips for improving Kavana (concentration during service)

There are 2 main difficulties when it comes to prayer, concentration and understanding/feeling. You seem to be asking for advice on understanding/feeling, so I’ll share what I found has worked for me.

First of all, the best way to get what you want, is to pray. Praying for God to help you connect to the prayers is no different.
I’d add to this, that customizing a little prayer (asking for his help in this next endeavor) for right before you pray can put someone in a great state of mind, kind of like how many people say a prayer before they begin learning. I’ve found that they both give me extra confidence.

Second, instead of having in mind something you currently need (which I think is often the tendency of people), try going in to the prayers with an image of who you want to be and how you want your life to look. (The reason I say this is because I think it’s natural to feel some insecurity when praying to God for things you need now. I think approaching it in the form of a bigger plan, like preparing for everything I need in life, gives a structured and more genuine feel to it.) Then as you go through all the prayers, think about how without each and every one of them, your goals and dreams are completely out of reach. Best case scenario is you’ll feel or at least recognize a sense of desperation.

I find that achieving a sense of desperation when praying is conducive for kavana (maybe it’s even the main point of kavana). You kind of just get naturally sucked in and start to feel the profundity of even the smallest prayers. (After all, whether we know it or not, we’re all a little desperate).

If you can master the art of desperation, then just take all those prayers draped in desperation, and send them up to God. He’d like that.

Good luck

Edit: if there are any seforim that mention any of these ideas I’d love for someone to let me know in the comments.

Based on your question, this sounds like a good candidate for a community wiki answer. All are welcome to add their personal tips

• I'm not the kinda guy you would think of as being into "meditation," but fact is, I find that the classic technique of paying attention to your breath and body to really help myself focus. I admit that sometimes I am spacing out and forget to do this, but usually I take a deep breath as I make my three steps back (before the Amidah), hold it in for a bit while either waiting for the Hazzan or whatever, and exhale very slowly while taking my three steps forward. I focus on the breath as it leaves my mouth, and this helps me focus on the words א-דני שפתי תפתח and/or כי שם ה' אקרא. Whenever I find my mind wandering while saying the words of the Amidah, I stop, and take a few very slow breaths, really focusing on the air going in and out, and it helps me think of my life, how fragile it is and so dependent upon God, and brings me back to the words I'm supposed to be saying.
• There are halakhic guidelines to how to hold your hands, your siddur, what to look at, etc., and so even though this might not be the halakhic "ideal," I find it helpful to very intentionally switch it up every few months: I'll spend a few months using the same siddur every day, then a few months praying by heart with my eyes closed, then use a new siddur, etc. I've been doing this for about 15 years and I find that it helps a lot so that I never get too used to the particular formatting of where words are on a page and the like, and this way my brain is still 'working' while I pray.
• Sometimes (see above), I have highlighted the names of God in my siddur so that when my eyes/mind see the different text color, I refocus.