Option 1 - "Set and forget" - Pick one expensive regular goalkeeper that you will never change out (Ederson/Alisson/De Gea), and a 4.0 million keeper that stays permanently on your bench.
Option 2 - Rotation - Pick two 4.5/5.0 million rated keepers, and rotate based on the best fixture for each. Teams lower down may not keep many clean sheets, but the goalies can pick up extra points for the amount of saves they have to make.
Invest in attacking defenders (wing backs, centre backs who come up for corners). Defenders get more points per goal/assist than midfielders or strikers, and the best value in terms of points per million will come from these players. The Liverpool defenders look super expensive, but someone like Trent Alexander Arnold (7.0) could easily end up getting as many points in defence from assists as De Bruyne (9.5) does as a midfielder.
Defenders do get points for clean sheets, however even the best teams concede regularly, so focus on attacking returns from these positions.
Having said that, there are cheaper players from the big teams, who aren't attacking, but might end up featuring regularly and get you more consistent points from clean sheets (Calum Chambers, Kurt Zouma, Axel Tuanzebe, etc)
Midfield and Attack
Try to ensure you have one premium option in each of these positions (Salah, Sterling, Aguero, Kane). It is easier to downgrade from these if they are not performing, than it is to upgrade from a squad of average players if one of thes top guys hits incredible form.
Never go for defensive midfielders (Jorginho, Matic, Kante, Fernandinho). They are a cheap for a reason. Midfield onwards is all about attacking returns, and these guys do not get goals or assists.
Other Squad Tips
Look out for players labelled out of position. Lundstram (4.0) is listed as a Defender, but seems to be a starting midfielder. This makes him insanely good value if he can be as creative as we all know he can be. Similarly, Perez at Leicester is pretty much a striker, but is listed in Midfield on the game. The further back the pitch a player is listed, the more points they will get for their attacking returns.
You get one free transfer a week. Any more than this incurs a penalty of -4 points per additional transfer.
This rolls over for one week, so if you do not make a transfer one week, you can make 2 transfers the next week. However it never rolls over past 2 free transfers, so don't wait more than that to make changes if necessary.
The value of your players can rise and fall based on how many other players start transferring them in or out. For every 0.2 million in value your players gain, you will only get 0.1 of the value back if you transfer them out of your team.
Bench Boost - All players in your squad earn points towards your total, not just the starting XI
Free Hit - Opportunity to switch out your whole team for one gameweek. NOTE: The squad will revert back to the previously held players the following week.
Triple Captain - Your captain earns 3x the weekly points, instead of 2x
Wildcard - Make unlimited transfers without any points deductions. Play this before January, as this resets in time for the winter transfer window, and you will have another available to play at any point in the second half of the campaign
Most people to play these are during Double Gameweeks towards the end of the season, where teams have a midweek and weekend PL match that both come under the umbrella of one gameweek. So for instance, your Triple Captain could earn treble points across two games, or you could use a free hit to select all players that are playing twice that week.
NOTE: You cannot use more than one chip in a gameweek, so make sure you do not let these all run down to the last week of the season.
If you're struggling for bargains to fill out your team, here are some I'd recommend monitoring the below players. I can't guarantee major points, but they should play regularly, and guarantee some points for their value.
Maitland Niles (5.0)