How to become a better voice-over artist
I think the main thing here is not to forget that whatever language/accent you do for voice work, you are still acting. Every voice job you do - consider it an acting performance. And for that - it's important to work on the tone, accent, character, etc. even more, because the people can't see you.
Getting a voice coach that you can trust is key. it's not about getting the most expensive one, it's about getting the one that helps you progress in the direction you need to. trust is extremely important.
Do warm ups every day. As any other muscle, your tongue, jaws and lips need to be warmed up and ready to work.
Practice accents and voices every day. For accents: record the accent coaching sessions and listen back to them, watch programs that use that accents. Try to 'vet' your accent by a native accent speaker: you are learning Swedish accent, try and find an actual Swedish person who speaks good English to help: friends of friends, Facebook groups, acting community, etc.
For voices: record them and listen back, they must not sound forced or as a joke. And that simply means - be the character you are doing the voice for, remember your acting.
Do not go out drinking the night before a recording session, this will affect your voice. If you have a cold or blocked nose, drink hot liquids and try to breathe through the nose as much as you can, use Vicks spray, but don't overuse it as that will affect your voice. A table spoonful of warm apple cider vinegar has been known as a 'voice fixer'.
Remember, dairy products affect the vocal chords, they create mucus that can also affect your voice.
Always have a glass of water when recording and drink as often as you can.